Take advantage of ARM unaligned memory access while writing clean C code

Question

It used to be that ARM processors were unable to properly handle unaligned memory access (ARMv5 and below). Something like u32 var32 = *(u32*)ptr; would just fail (raise exception) if ptr was not properly aligned on 4-bytes.

Writing such a statement would work fine for x86/x64 though, since these CPU have always handled such situation very efficiently. But according to C standard, this is not a "proper" way to write it. u32 is apparently equivalent to a structure of 4 bytes which must be aligned on 4 bytes.

A proper way to achieve the same result while keeping the orthodoxy correctness and ensuring full compatibility with any cpu is :

u32 read32(const void* ptr) 
{ 
    u32 result; 
    memcpy(&result, ptr, 4); 
    return result; 
}

This one is correct, will generate proper code for any cpu able or not to read at unaligned positions. Even better, on x86/x64, it's properly optimized to a single read operation, hence has the same performance as the first statement. It's portable, safe, and fast. Who can ask more ?

Well, problem is, on ARM, we are not so lucky.

Writing the memcpy version is indeed safe, but seems to result in systematic cautious operations, which are very slow for ARMv6 and ARMv7 (basically, any smartphone).

In a performance oriented application which heavily relies on read operations, the difference between the 1st and 2nd version could be measured : it stands at > 5x at gcc -O2 settings. This is way too much to be ignored.

Trying to find a way to use ARMv6/v7 capabilities, I've looked for guidance on a few example codes around. Unfortunatley, they seem to select the first statement (direct u32 access), which is not supposed to be correct.

That's not all : new GCC versions are now trying to implement auto-vectorization. On x64, that means SSE/AVX, on ARMv7 that means NEON. ARMv7 also supports some new "Load Multiple" (LDM) and "Store Multiple" (STM) opcodes, which require pointer to be aligned.

What does that mean ? Well, the compiler is free to use these advanced instructions, even if they were not specifically called from the C code (no intrinsic). To take such decision, it uses the fact the an u32* pointer is supposed to be aligned on 4 bytes. If it's not, then all bets are off : undefined behavior, crashes.

What that means is that even on CPU which support unaligned memory access, it's now dangerous to use direct u32 access, as it can lead to buggy code generation at high optimization settings (-O3).

So now, this is a dilemna : how to access the native performance of ARMv6/v7 on unaligned memory access without writing the incorrect version u32 access ?

PS : I've also tried __packed() instructions, and from a performance perspective, they seem to work exactly the same as the memcpy method.

[Edit] : Thanks for the excellent elements received so far.

Looking at the generated assembly, I could confirm @Notlikethat finding that memcpy version does indeed generate proper ldr opcode (unaligned load). However, I also found that the generated assembly uselessly invokes str (command). So the complete operation is now an unaligned load, an aligned store, and then a final aligned load. That's a lot more work than necessary.

Answering @haneefmubarak, yes the code is properly inlined. And no, memcpy is very far from providing the best possible speed, since forcing the code to accept direct u32 access translates into huge performance gains. So some better possibility must exist.

A big thank to @artless_noise. The link to godbolt service is unvaluable. I've never been able to see so clearly the equivalence between a C source code and its assembly representation. This is highly inspiring.

I completed one of @artless examples, and it gives the following :

#include <stdlib.h>
#include <memory.h>
typedef unsigned int u32;

u32 reada32(const void* ptr) { return *(const u32*) ptr; }

u32 readu32(const void* ptr) 
{ 
    u32 result; 
    memcpy(&result, ptr, 4); 
    return result; 
}

once compiled using ARM GCC 4.8.2 at -O3 or -O2 :

reada32(void const*):
    ldr r0, [r0]
    bx  lr
readu32(void const*):
    ldr r0, [r0]    @ unaligned
    sub sp, sp, #8
    str r0, [sp, #4]    @ unaligned
    ldr r0, [sp, #4]
    add sp, sp, #8
    bx  lr

Quite telling ....

Answer 1

OK, the situation is more confusing than one would like. So, in an effort to clarify, here are the findings on this journey :

accessing unaligned memory

1. The only portable C standard solution to access unaligned memory is the memcpy one. I was hoping to get another one through this question, but apparently it's the only one found so far.

Example code :

u32 read32(const void* ptr)  { 
    u32 value; 
    memcpy(&value, ptr, sizeof(value)); 
    return value;  }

This solution is safe in all circumstances. It also compiles into a trivial load register operation on x86 target using GCC.

However, on ARM target using GCC, it translates into a way too large and useless assembly sequence, which bogs down performance.

Using Clang on ARM target, memcpy works fine (see @notlikethat comment below). It would be easy to blame GCC at large, but it's not that simple : the memcpy solution works fine on GCC with x86/x64, PPC and ARM64 targets. Lastly, trying another compiler, icc13, the memcpy version is surprisingly heavier on x86/x64 (4 instructions, while one should be enough). And that's just the combinations I could test so far.

I have to thank godbolt's project to make such statements easy to observe.

2. The second solution is to use __packed structures. This solution is not C standard, and entirely depends on compiler's extension. As a consequence, the way to write it depends on the compiler, and sometimes on its version. This is a mess for maintenance of portable code.

That being said, in most circumstances, it leads to better code generation than memcpy. In most circumstances only ...

For example, regarding the above cases where memcpy solution does not work, here are the findings :

• on x86 with ICC : __packed solution works
• on ARMv7 with GCC : __packed solution works

• on ARMv6 with GCC : does not work. Assembly looks even uglier than memcpy.

  3. The last solution is to use direct u32 access to unaligned memory positions. This solution used to work for decades on x86 cpus, but is not recommended, as it violates some C standard principles : compiler is authorized to consider this statement as a guarantee that data is properly aligned, leading to buggy code generation.

Unfortunately, in at least one case, it is the only solution able to extract performance from target. Namely for GCC on ARMv6.

Do not use this solution for ARMv7 though : GCC can generate instructions which are reserved for aligned memory accesses, namely LDM (Load Multiple), leading to crash.

Even on x86/x64, it becomes dangerous to write your code this way nowadays, as the new generation compilers may try to auto-vectorize some compatible loops, generating SSE/AVX code based on the assumption that these memory positions are properly aligned, crashing the program.

As a recap, here are the results summarized as a table, using the convention : memcpy > packed > direct.

| compiler  | x86/x64 | ARMv7  | ARMv6  | ARM64  |  PPC   |
|-----------|---------|--------|--------|--------|--------|
| GCC 4.8   | memcpy  | packed | direct | memcpy | memcpy |
| clang 3.6 | memcpy  | memcpy | memcpy | memcpy |   ?    |
| icc 13    | packed  | N/A    | N/A    | N/A    | N/A    |