[llvm-dev] A pattern for portable __builtin_add_overflow()

[Paweł Bylica via llvm-dev]

Thanks Sanjoy,

It looks it works only up to 32-bit integers (
http://llvm.org/doxygen/InstCombineCompares_8cpp_source.html#l01255).
I will need it also for 64-bit integers.

Even if the pattern match would handle the 64-bit case I would need to use
128-bit types what is not portable still.

// P.

On Thu, Nov 22, 2018 at 6:54 PM Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> Going by InstCombiner::foldICmpWithConstant,
> https://gcc.godbolt.org/z/To_qmm should work.
>
> -- Sanjoy
> On Tue, Nov 20, 2018 at 2:46 AM Paweł Bylica via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi LLVM, clang,
> >
> > I'm trying to write a portable version of __builtin_add_overflow() it a
> way that the compiler would
> > recognize the pattern and use the add_overflow intrinsic / the best
> possible machine instruction.
> >
> > Here are docs about these builtins:
> https://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins
> .
> >
> > With unsigned types this is easy:
> >
> > int uaddo_native(unsigned a, unsigned b, unsigned* s)
> > {
> > return __builtin_add_overflow(a, b, s);
> > }
> >
> > int uaddo_portable(unsigned a, unsigned b, unsigned* s)
> > {
> > *s = a + b;
> > return *s < a;
> > }
> >
> > We get exactly the same assembly:
> > uaddo_native: # @uaddo_native
> > xor eax, eax
> > add edi, esi
> > setb al
> > mov dword ptr [rdx], edi
> > ret
> > uaddo_portable: # @uaddo_portable
> > xor eax, eax
> > add edi, esi
> > setb al
> > mov dword ptr [rdx], edi
> > ret
> >
> > But with signed types it is not so easy. I tried 2 versions, but the
> result is quite far away from the optimal assembly.
> >
> >
> > int saddo_native(int a, int b, int* s)
> > {
> > return __builtin_add_overflow(a, b, s);
> > }
> >
> > int saddo_portable(int a, int b, int* s)
> > {
> > *s = (unsigned)a + (unsigned)b;
> > return (a > 0) ? *s <= b : *s > b;
> > }
> >
> > int saddo_portable2(int a, int b, int* s)
> > {
> > *s = (unsigned)a + (unsigned)b;
> > int cond = a > 0;
> > int check = *s > b;
> > return (cond & !check) | (!cond & check);
> > }
> >
> > Assembly:
> >
> > saddo_native: # @saddo_native
> > xor eax, eax
> > add edi, esi
> > seto al
> > mov dword ptr [rdx], edi
> > ret
> > saddo_portable: # @saddo_portable
> > lea eax, [rsi + rdi]
> > mov dword ptr [rdx], eax
> > cmp eax, esi
> > setle al
> > setg cl
> > test edi, edi
> > jg .LBB3_2
> > mov eax, ecx
> > .LBB3_2:
> > movzx eax, al
> > ret
> > saddo_portable2: # @saddo_portable2
> > lea eax, [rsi + rdi]
> > mov dword ptr [rdx], eax
> > test edi, edi
> > setg cl
> > cmp eax, esi
> > setg al
> > xor al, cl
> > movzx eax, al
> > ret
> >
> >
> > Do you know the trick to force the compiler to use the seto instruction?
> >
> > I also noticed that the transformation for uaddo_portable happens in
> CodeGen, not in IR.
> >
> > Bests,
> > Paweł
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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