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