Fix CMSIS-NN 7.0 Impossible Constraints Error

Aug 5, 2025 by Felix Dubois 48 views

Resolving "Impossible Constraints" Error Compiling CMSIS-NN 7.0 with arm-none-eabi-gcc 13.2

Introduction

Hey everyone! Today, we're diving deep into a tricky issue encountered while compiling CMSIS-NN 7.0 using arm-none-eabi-gcc 13.2. Specifically, we'll be dissecting the dreaded "impossible constraints" error that popped up when targeting the M55 platform. This can be a real head-scratcher, especially when you're trying to optimize your neural network kernels for embedded systems. If you've been wrestling with this error, you're definitely in the right place. We'll break down the problem, explore potential causes, and, most importantly, provide solutions to get your compilation back on track. So, buckle up and let's get started on this journey to resolve this pesky compilation error!

When working with embedded systems and neural networks, optimizing performance is crucial. CMSIS-NN, a collection of efficient neural network kernels developed by ARM, plays a vital role in achieving this optimization. However, sometimes, the path to high performance is paved with cryptic error messages. One such message, the "impossible constraints" error, can arise when compiling CMSIS-NN, particularly with newer versions of the GNU Arm Embedded Toolchain. This article aims to shed light on this issue, specifically in the context of CMSIS-NN 7.0 and arm-none-eabi-gcc 13.2, and provide a comprehensive guide to resolving it. We will explore the root causes of the error, examine the compilation parameters that trigger it, and offer practical solutions to overcome this hurdle. This guide is designed to help developers, engineers, and enthusiasts who are working on embedded machine learning applications and are seeking to optimize their neural network implementations. By understanding the intricacies of the compiler and the CMSIS-NN library, we can effectively tackle this error and unlock the full potential of our embedded systems. Remember, the goal is not just to fix the error, but to understand why it occurs and how to prevent it in the future. So, let's dive in and unravel the mystery behind the "impossible constraints" error in CMSIS-NN compilation. Throughout this article, we will use real-world examples and code snippets to illustrate the concepts and solutions, making it easier for you to apply them to your own projects. Let's get started and conquer this challenge together!

The Problem: Impossible Constraints Error

Let's get straight to the heart of the matter. The error we're tackling looks something like this:

In file included from /xxx/CMSIS_5/CMSIS/NN/Include/arm_nnsupportfunctions.h:33,
                 from /xxx/CMSIS_5/CMSIS/NN/Source/NNSupportFunctions/arm_nn_mat_mul_core_4x_s8.c:30:
/xxx/CMSIS_5/CMSIS/NN/Source/NNSupportFunctions/arm_nn_mat_mul_core_4x_s8.c: In function 'arm_nn_mat_mul_core_4x_s8':
/xxx/CMSIS_5/CMSIS/NN/Include/Internal/arm_nn_compiler.h:97:23: error: 'asm' operand has impossible constraints
   97 |         #define __ASM __asm
      |                       ^~~~~
/xxx/CMSIS_5/CMSIS/NN/Source/NNSupportFunctions/arm_nn_mat_mul_core_4x_s8.c:84:9: note: in expansion of macro '__ASM'
   84 |         __ASM volatile(" .p2align 2                             \n"
      |         ^~~~~

This error, "'asm' operand has impossible constraints," is a classic sign that the compiler is struggling to reconcile the inline assembly code within CMSIS-NN with the compilation flags you've set. Specifically, it often points to a mismatch between the hardware architecture, the compiler's expectations, and the assembly instructions used in the CMSIS-NN library. The error message itself can be quite cryptic, leaving you wondering where to even begin troubleshooting. But don't worry, we're here to demystify it. The key thing to understand is that this error arises during the assembly phase of compilation, where the compiler is trying to translate your C code, along with the inline assembly in CMSIS-NN, into machine code for your target processor. When the compiler encounters an instruction or a set of instructions that it cannot map to the available registers or memory locations, it throws this "impossible constraints" error. This typically happens when the constraints specified in the inline assembly code are not compatible with the target architecture or the current compiler settings. To effectively resolve this issue, we need to dig deeper into the compilation parameters, the target architecture, and the specific assembly code within CMSIS-NN that is causing the problem. In the following sections, we will dissect each of these aspects to gain a clearer understanding of the error and how to fix it. So, let's move on to the next section and start exploring the potential causes of this error.

Compilation Parameters Triggering the Error

Now, let's talk about the compilation parameters. These are the flags you pass to the compiler that tell it how to build your code. In this case, the flags that seem to be stirring up trouble are:

-mtune=cortex-m55 -march=armv8.1-m.main+dsp+mve.fp+fp.dp -mfpu=fpv5-d16 -mfloat-abi=hard -O3

Let's break these down:

-mtune=cortex-m55: This tells the compiler to optimize the generated code for the Cortex-M55 processor. This is a good thing in general, as it allows the compiler to make architecture-specific optimizations. However, it also means the compiler will assume certain architectural features are available.
-march=armv8.1-m.main+dsp+mve.fp+fp.dp: This flag specifies the target architecture. Here, we're targeting ARMv8.1-M with DSP, MVE (Helium) floating-point support (both single-precision and double-precision). This is a crucial flag because it dictates which instructions the compiler is allowed to use.
-mfpu=fpv5-d16: This option specifies the floating-point unit (FPU) architecture. fpv5-d16 indicates a single-precision FPU with 16 double-precision registers. This is compatible with the armv8.1-m.main architecture.
-mfloat-abi=hard: This flag specifies the floating-point ABI (Application Binary Interface). hard means that floating-point operations are performed directly using hardware FPU instructions, which is generally more efficient but requires an FPU to be present.
-O3: This is the optimization level. -O3 tells the compiler to perform aggressive optimizations, which can sometimes expose subtle bugs or lead to unexpected behavior if the code isn't perfectly written. While optimization is generally beneficial, it can also exacerbate issues related to inline assembly and register allocation.

The combination of these flags tells the compiler that we're targeting a Cortex-M55 with advanced features like DSP and MVE, and that we want the code to be highly optimized. While this is a common and often desirable setup for embedded machine learning, it also sets the stage for potential conflicts with the inline assembly within CMSIS-NN. The compiler, armed with these flags, makes certain assumptions about the available registers and instruction sets. If the inline assembly in CMSIS-NN doesn't align with these assumptions, we run into the "impossible constraints" error. Specifically, the compiler might try to use registers or instructions that are either not available or are being used in a way that conflicts with the assembly code. In the next section, we'll delve into the specific assembly code within CMSIS-NN that's causing the issue and explore why these flags might be triggering the error. Understanding the interplay between the compilation parameters and the assembly code is key to finding the right solution.

Examining the Offending Code in `arm_nn_mat_mul_core_4x_s8.c`

Alright, let's put on our detective hats and dive into the arm_nn_mat_mul_core_4x_s8.c file, where the error originates. This file contains a crucial function for performing matrix multiplication, a fundamental operation in neural networks. The error message points us to line 84, which contains the following inline assembly block:

__ASM volatile(" .p2align 2                             \n"
               "                                           \n"
               "    ldr     r0, [%[inputA], :sxtw #0]!    \n"
               "    ldr     r1, [%[inputA], :sxtw #0]!    \n"
               "    ldr     r2, [%[inputA], :sxtw #0]!    \n"
               "    ldr     r3, [%[inputA], :sxtw #0]!    \n"
               "                                           \n"
               "    smlabb  %[sum0], r0, r4, %[sum0]       \n"
               "    smlabb  %[sum1], r0, r5, %[sum1]       \n"
               "    smlabb  %[sum2], r0, r6, %[sum2]       \n"
               "    smlabb  %[sum3], r0, r7, %[sum3]       \n"
               "                                           \n"
               "    smlabb  %[sum0], r1, r4, %[sum0]       \n"
               "    smlabb  %[sum1], r1, r5, %[sum1]       \n"
               "    smlabb  %[sum2], r1, r6, %[sum2]       \n"
               "    smlabb  %[sum3], r1, r7, %[sum3]       \n"
               "                                           \n"
               "    smlabb  %[sum0], r2, r4, %[sum0]       \n"
               "    smlabb  %[sum1], r2, r5, %[sum1]       \n"
               "    smlabb  %[sum2], r2, r6, %[sum2]       \n"
               "    smlabb  %[sum3], r2, r7, %[sum3]       \n"
               "                                           \n"
               "    smlabb  %[sum0], r3, r4, %[sum0]       \n"
               "    smlabb  %[sum1], r3, r5, %[sum1]       \n"
               "    smlabb  %[sum2], r3, r6, %[sum2]       \n"
               "    smlabb  %[sum3], r3, r7, %[sum3]       \n"
               : [sum0] "+r" (sum_0), [sum1] "+r" (sum_1), [sum2] "+r" (sum_2), [sum3] "+r" (sum_3)
               : [inputA] "r" (input_a), "r" (ker_0), "r" (ker_1), "r" (ker_2), "r" (ker_3)
               : "r0", "r1", "r2", "r3", "cc");

This block of assembly code is hand-optimized for matrix multiplication on ARM processors. It uses the smlabb instruction, which performs a signed multiply-accumulate operation, and loads data using ldr instructions. The key here is the constraint string in the assembly block. Constraint strings tell the compiler which registers or memory locations the assembly code needs to access. In this case, we see constraints like "+r" and "r", which indicate that the compiler can use any general-purpose register for the corresponding variables. The issue arises when the compiler, given the -O3 optimization level and the target architecture flags, tries to allocate registers in a way that conflicts with these constraints. For instance, the compiler might decide to use a register that's already being used for something else, or it might try to use a register that's not compatible with the smlabb instruction. The "impossible constraints" error essentially means that the compiler has exhausted its options and can't find a valid register allocation that satisfies both the assembly code's requirements and the overall program's needs. This situation is more likely to occur with higher optimization levels, as the compiler becomes more aggressive in its register allocation strategies. To resolve this, we need to either guide the compiler towards a better register allocation or modify the assembly code to be more flexible. In the following sections, we will explore several solutions that address these issues, including modifying the constraint strings, disabling certain optimizations, or even rewriting the assembly code. By understanding the constraints and the compiler's behavior, we can effectively tame this error and achieve our desired performance.

Solutions to the Impossible Constraints Error

Okay, enough with the problem – let's talk solutions! Here are a few strategies you can use to tackle this "impossible constraints" error. We'll start with the simplest and most common fixes and then move on to more advanced techniques if needed.

1. Modifying the Constraint Strings

The constraint strings in the inline assembly are the first place to look. These strings tell the compiler which registers are allowed for each operand. Sometimes, the compiler gets confused by overly general constraints like "r" (which means "any general-purpose register"). We can try to be more specific and suggest particular registers. However, this approach should be used with caution, as it can lead to code that is less portable and harder to maintain. A safer approach is to ensure that the output operands are properly marked as read-write using the "+r" constraint, as seen in the original code. This tells the compiler that the register will be both read from and written to, which is crucial for accumulator variables like sum0, sum1, sum2, and sum3. Another potential issue is the number of input operands. The compiler has a limited number of registers to work with, and if the assembly code requires too many registers, it can lead to constraint conflicts. In such cases, we can try to reduce the number of input operands by reusing registers or by loading values into registers outside the assembly block. However, this might impact performance, so it's a trade-off that needs to be carefully considered. In the context of the given assembly code, a common approach is to examine the registers used for the input pointers (input_a, ker_0, ker_1, ker_2, ker_3) and the accumulator variables (sum_0, sum_1, sum_2, sum_3). If there are any potential overlaps or conflicts, we can try to explicitly assign different registers to these variables. This can be done by using register names directly in the constraint string, such as "r0", "r1", etc. However, this makes the code less portable and harder to maintain, as the register assignments are fixed and cannot be changed by the compiler. Therefore, it's generally better to avoid hardcoding register names unless absolutely necessary. Instead, we should focus on using more general constraints and rely on the compiler to make the best register allocation decisions. If the "impossible constraints" error persists, it might indicate a deeper issue with the assembly code or the compiler's optimization strategies. In such cases, we might need to explore other solutions, such as disabling certain optimizations or rewriting the assembly code. However, modifying the constraint strings is a good first step, as it often resolves the issue without requiring more drastic changes.

2. Reducing the Optimization Level

Sometimes, the -O3 optimization level is just too aggressive. The compiler tries to be too clever, and it ends up painting itself into a corner. Try reducing the optimization level to -O2 or even -O1. This gives the compiler more freedom in register allocation and can often resolve the issue. Reducing the optimization level can have a significant impact on the generated code. At -O3, the compiler performs extensive optimizations, such as loop unrolling, function inlining, and instruction scheduling, to maximize performance. While these optimizations can lead to substantial speed improvements, they also increase the complexity of the code and the demands on the compiler's register allocation capabilities. By reducing the optimization level to -O2 or -O1, we tell the compiler to be less aggressive in its optimizations. This can simplify the code and make it easier for the compiler to allocate registers without conflicts. However, it also means that the generated code might be less performant. Therefore, reducing the optimization level is a trade-off between compilation success and runtime performance. In some cases, the performance difference between -O3 and -O2 might be negligible, especially for small or medium-sized projects. In other cases, the performance drop might be significant, particularly for computationally intensive tasks like matrix multiplication. To determine the optimal optimization level, it's essential to benchmark the code at different optimization levels and measure the performance impact. This will help us find the sweet spot where the code compiles without errors and still delivers acceptable performance. In addition to reducing the overall optimization level, we can also try disabling specific optimizations that are known to cause issues. For example, the -fomit-frame-pointer optimization, which removes the frame pointer register to free up an additional register, can sometimes interfere with inline assembly code. Disabling this optimization by adding -fno-omit-frame-pointer to the compilation flags can resolve the "impossible constraints" error in some cases. However, disabling specific optimizations should be done with caution, as it can have unintended consequences on the performance and stability of the code. It's always best to understand the purpose of each optimization and its potential impact before disabling it. In summary, reducing the optimization level is a simple and effective way to address the "impossible constraints" error, but it's important to consider the performance implications and benchmark the code at different levels to find the optimal balance.

3. Rewriting the Assembly Code (If Necessary)

If the above solutions don't work, you might need to get your hands dirty and rewrite the assembly code. This is the most complex solution, but it gives you the most control. The goal here is to make the assembly code more flexible and less demanding in terms of register usage. Rewriting assembly code is a delicate process that requires a deep understanding of the target architecture, the compiler's behavior, and the specific requirements of the code. It's not a task to be taken lightly, as even a small mistake can lead to subtle bugs or performance regressions. However, in some cases, it's the only way to resolve the "impossible constraints" error and achieve the desired performance. When rewriting assembly code, the first step is to carefully analyze the existing code and identify the areas that are causing the register allocation conflicts. This might involve examining the constraint strings, the instruction sequences, and the data dependencies. Once we have a clear understanding of the problem, we can start exploring alternative ways to implement the same functionality. One common technique is to reduce the number of registers used by the assembly code. This can be done by reusing registers, spilling registers to memory, or using alternative instructions that require fewer registers. However, these techniques can also impact performance, so it's important to benchmark the code after making any changes. Another approach is to make the assembly code more flexible in terms of register usage. This can be achieved by using more general constraint strings or by allowing the compiler to allocate registers dynamically. However, this requires a deeper understanding of the compiler's register allocation algorithms and can be more challenging to implement. In the context of the arm_nn_mat_mul_core_4x_s8.c file, we might consider rewriting the assembly code to use a different matrix multiplication algorithm or to optimize the data loading and storing operations. For example, we could try using the MVE (Helium) instructions available on the Cortex-M55 processor to perform the matrix multiplication more efficiently. However, this would require a significant rewrite of the assembly code and a thorough understanding of the MVE instruction set. Before embarking on a major rewrite of the assembly code, it's important to consider the maintainability and portability of the code. Hand-optimized assembly code can be difficult to understand and maintain, and it might not be portable to other architectures or compilers. Therefore, it's often a good idea to explore alternative solutions, such as using compiler intrinsics or libraries, before resorting to rewriting assembly code. In summary, rewriting assembly code is a powerful but complex solution to the "impossible constraints" error. It requires a deep understanding of the target architecture, the compiler's behavior, and the specific requirements of the code. It's important to carefully analyze the existing code, explore alternative implementations, and benchmark the code after making any changes. And remember, maintainability and portability should always be considered.

4. Disabling MVE (Helium) Instructions (If Not Required)

Since the -march flag includes +mve.fp+fp.dp, the compiler might be trying to use MVE instructions. If you don't actually need MVE, try removing this part of the flag. This can simplify the code generation and potentially avoid the register allocation issues. MVE, or Helium, is ARM's vector processing technology for the Cortex-M family of microcontrollers. It provides a set of SIMD (Single Instruction, Multiple Data) instructions that can significantly accelerate signal processing and machine learning tasks. However, MVE instructions also have specific register requirements and can complicate the compiler's register allocation process. If your application doesn't heavily rely on MVE instructions, disabling them can simplify the code generation and potentially resolve the "impossible constraints" error. This is particularly true if the error occurs in code that doesn't explicitly use MVE instructions, as the compiler might be trying to use them for optimization purposes even if they are not strictly necessary. To disable MVE, you can modify the -march flag to exclude the +mve.fp+fp.dp option. For example, you could change the flag from -march=armv8.1-m.main+dsp+mve.fp+fp.dp to -march=armv8.1-m.main+dsp. This tells the compiler to target the ARMv8.1-M architecture with DSP extensions but without MVE support. After making this change, it's important to recompile your code and test it thoroughly to ensure that it still functions correctly. Disabling MVE can impact the performance of certain operations, particularly those that benefit from vector processing. Therefore, it's essential to benchmark your code before and after disabling MVE to assess the performance impact. If you find that disabling MVE significantly degrades performance, you might need to explore other solutions, such as rewriting the assembly code to use MVE instructions more efficiently or adjusting the compiler's optimization settings. In some cases, it might be possible to selectively disable MVE for specific parts of your code while keeping it enabled for other parts. This can be achieved by using compiler attributes or pragmas to control the instruction set used for different functions or code blocks. However, this requires a deeper understanding of the compiler's behavior and can be more challenging to implement. In summary, disabling MVE is a relatively simple way to potentially resolve the "impossible constraints" error, but it's important to consider the performance implications and test your code thoroughly. If your application doesn't heavily rely on MVE instructions, disabling them can simplify the code generation and avoid register allocation conflicts. However, if MVE is crucial for your application's performance, you might need to explore other solutions.

Conclusion

The "impossible constraints" error when compiling CMSIS-NN can be a frustrating obstacle, but it's not insurmountable. By understanding the interplay between compilation flags, assembly code, and compiler behavior, you can systematically diagnose and resolve the issue. We've explored several solutions, from tweaking constraint strings to rewriting assembly code, giving you a toolkit to tackle this problem. Remember, the key is to approach the problem methodically, testing each solution to see if it resolves the error without introducing new issues. And as always, don't hesitate to consult the compiler documentation and online resources for further guidance. Happy compiling, folks!

In conclusion, the "impossible constraints" error in CMSIS-NN compilation is a common but challenging issue that can arise when working with embedded systems and neural networks. It's often caused by a mismatch between the compilation parameters, the assembly code, and the compiler's register allocation strategies. However, by understanding the root causes of the error and applying the solutions outlined in this article, you can effectively overcome this obstacle and achieve your desired performance. We've covered a range of solutions, from modifying the constraint strings and reducing the optimization level to rewriting the assembly code and disabling MVE instructions. Each solution has its own trade-offs, and the best approach depends on the specific context of your project. The key is to approach the problem systematically, testing each solution to see if it resolves the error without introducing new issues. Remember, the goal is not just to fix the error, but to understand why it occurs and how to prevent it in the future. This requires a deep understanding of the target architecture, the compiler's behavior, and the specific requirements of your code. By investing the time and effort to gain this understanding, you'll be well-equipped to tackle any compilation challenges that come your way. And as always, don't hesitate to consult the compiler documentation, online resources, and the CMSIS-NN community for further guidance and support. The world of embedded machine learning is constantly evolving, and there's always something new to learn. So, keep exploring, keep experimenting, and keep pushing the boundaries of what's possible. Happy coding, and may your compilations be error-free!