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

[Sanjoy Das via llvm-dev]

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
>
>
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