llvm dev - Aug 2019 - Suboptimal code generated by clang+llc in quite a common scenario (?)

This might not be the workaround you want because it is only available in
C, but you can use restrict to allow such optimizations.

https://godbolt.org/z/2gQ26f

Alex


On Thu, Aug 8, 2019 at 11:50 AM Michael Kruse via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> 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
> >
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Thu, 8 Aug 2019 at 17:08, Alex Brachet-Mialot via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> This might not be the workaround you want because it is only available in
C, but you can use restrict to allow such optimizations.
It also doesn't work in Clang unfortunately. We can only represent
restrict on function arguments at the moment.

Cheers.

Tim.

Hi Tim and Alex

Thanks for your replies. 

So just to make it clear for me: does this imply that there’s indeed no way on
the current version to tell the compiler or Clang to optimize this?

Thanks,

Joan

> On 8 Aug 2019, at 18:30, Tim Northover via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> On Thu, 8 Aug 2019 at 17:08, Alex Brachet-Mialot via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> This might not be the workaround you want because it is only available
in C, but you can use restrict to allow such optimizations.
> 
> It also doesn't work in Clang unfortunately. We can only represent
> restrict on function arguments at the moment.
> 
> Cheers.
> 
> Tim.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hi Tim,

I wonder if you could help me with the following, even if just giving some
pointers about where to look. I previously posted a similar question in the
mailing list, but unfortunately I have not received a reply. This is the
subject:

I want to reduce the number of register spills to the stack that are created
around storeRegToStackSlot and loadRegFromStackSlot

In order to do so, I can use free registers from a second set of registers. The
idea is that these registers would be used as spills, instead of stack slots.
These registers may be free when they do not intervene in any instructions on a
given function.

I assumed that, by default, LLVM would figure out that there are free registers
(albeit from a different register class) and would use them as temporaries
instead of creating stack spills of the regular register set. However this is
not the case. The stack spills are created anyway despite being registers unused
on the second register bank.

I am unsure about why this happens or how to correct/implement this, or where to
look. Any pointers would be appreciated.

Thanks,

Joan
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