[llvm-dev] Suboptimal code generated by clang+llc in quite a common scenario (?)

[Michael Kruse via llvm-dev]

Hi,

char* scscx is an universal pointer and may point to anything,
including itself. That is, scscx might point to itself:

scscx = (char*)&scscx;

such that

scscx[0] = ...

changes the address scscx point to. A pointer to (int*) in contrast is
only allowed to point to integers in memory, it is not an universal
pointer. In particular, when accessing it the compiler can assume that
it is not aliasing with something that is of type char*.

For more details, see e.g. Wikipedia [1] or Stackoverflow [2]

[1] https://en.wikipedia.org/wiki/Pointer_aliasing#Aliasing_and_re-ordering
[2] https://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule

Michael


Am Do., 8. Aug. 2019 um 10:19 Uhr schrieb Joan Lluch via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> I found a something that I quite not understand when compiling a common piece of code using the -Os flags.
> I found it while testing my own backend but then I got deeper and found that at least the x86 is affected as well. This is the referred code:
>
> char pp[3];
> char *scscx = pp;
> int tst( char i, char j, char k )
> {
>   scscx[0] = i;
>   scscx[1] = j;
>   scscx[2] = k;
>   return 0;
> }
>
> The above gets compiled for the x86 architecture like this:
>
> ; Function Attrs: nofree norecurse nounwind optsize uwtable
> define i32 @tst(i8 signext %i, i8 signext %j, i8 signext %k) local_unnamed_addr #1 {
> entry:
>   %0 = load i8*, i8** @scscx, align 8, !tbaa !11
>   store i8 %i, i8* %0, align 1, !tbaa !13
>   %1 = load i8*, i8** @scscx, align 8, !tbaa !11
>   %arrayidx1 = getelementptr inbounds i8, i8* %1, i64 1
>   store i8 %j, i8* %arrayidx1, align 1, !tbaa !13
>   %2 = load i8*, i8** @scscx, align 8, !tbaa !11
>   %arrayidx2 = getelementptr inbounds i8, i8* %2, i64 2
>   store i8 %k, i8* %arrayidx2, align 1, !tbaa !13
>   ret i32 0
> }
>
> According to that, the variable ‘scscx’ is loaded three times despite it’s never modified. The resulting assembly code is this:
>
> .globl _tst
> _tst:
> .cfi_startproc
> pushl %ebp
> .cfi_def_cfa_offset 8
> .cfi_offset %ebp, -8
> movl %esp, %ebp
> .cfi_def_cfa_register %ebp
> pushl %esi
> .cfi_offset %esi, -12
> movb 16(%ebp), %al
> movb 12(%ebp), %cl
> movb 8(%ebp), %dl
> movl _scscx, %esi
> movb %dl, (%esi)
> movl _scscx, %edx
> movb %cl, 1(%edx)
> movl _scscx, %ecx
> movb %al, 2(%ecx)
> xorl %eax, %eax
> popl %esi
> popl %ebp
> retl
> .cfi_endproc
>
> .comm _pp,3,0
> .section __DATA,__data
> .globl _scscx
> .p2align 3
> _scscx:
> .long _pp
>
>
> Again, the _scscx is loaded three times instead of reusing a register, which is suboptimal.
>
>
> NOW, if I replace the original code by this:
>
> int pp[3];
> int *scscx = pp;
> int tst( int i, int j, int k )
> {
>   scscx[0] = i;
>   scscx[1] = j;
>   scscx[2] = k;
>   return 0;
> }
>
> I get the following:
>
>
> ; Function Attrs: nofree norecurse nounwind optsize uwtable
> define i32 @tst(i32 %i, i32 %j, i32 %k) local_unnamed_addr #1 {
> entry:
>   %0 = load i32*, i32** @scscx, align 8, !tbaa !11
>   store i32 %i, i32* %0, align 4, !tbaa !13
>   %arrayidx1 = getelementptr inbounds i32, i32* %0, i64 1
>   store i32 %j, i32* %arrayidx1, align 4, !tbaa !13
>   %arrayidx2 = getelementptr inbounds i32, i32* %0, i64 2
>   store i32 %k, i32* %arrayidx2, align 4, !tbaa !13
>   ret i32 0
> }
>
>
> .globl _tst
> _tst:
> .cfi_startproc
> pushl %ebp
> .cfi_def_cfa_offset 8
> .cfi_offset %ebp, -8
> movl %esp, %ebp
> .cfi_def_cfa_register %ebp
> pushl %esi
> .cfi_offset %esi, -12
> movl 16(%ebp), %eax
> movl 12(%ebp), %ecx
> movl 8(%ebp), %edx
> movl _scscx, %esi
> movl %edx, (%esi)
> movl %ecx, 4(%esi)
> movl %eax, 8(%esi)
> xorl %eax, %eax
> popl %esi
> popl %ebp
> retl
> .cfi_endproc
>
> .comm _pp,12,2
> .section __DATA,__data
> .globl _scscx
> .p2align 3
> _scscx:
> .long _pp
>
>
> In this case the compiler optimises the load of _scscx into a register and reuses its value instead of loading the variable multiple times. This results in a cleaner and more optimal code, specially when compared with the first case.
>
> I would like to understand why this happens, and whether there’s a way (or workaround) to improve it?
>
> Should I file a bug report for that?
>
> Thanks.
>
> Joan
>
>
>
>
>
>
>
>
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