https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67317
Bug ID: 67317
Summary: [x86] Silly code generation for
_addcarry_u32/_addcarry_u64
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: myriachan at gmail dot com
Target Milestone: ---
x86 intrinsics _addcarry_u32 and _addcarry_u64 generate silly code. For
example, the following function to get the result of a 64-bit addition (the XOR
is to the output clearer):
u64 testcarry(u64 a, u64 b, u64 c, u64 d)
{
u64 result0, result1;
_addcarry_u64(_addcarry_u64(0, a, c, &result0), b, d,
&result1);
return result0 ^ result1;
}
This is the code generated with -O1, -O2 and -O3:
xor r8d, r8d
add r8b, -1
adc rdx, rdi
setc r8b
mov rax, rdx
add r8b, -1
adc rcx, rsi
xor rax, rcx
ret
The first sillyness is that _addcarry_u64 does not optimize a compile-time
constant 0 being the first carry parameter. Instead of "adc", it should just
use "add".
The second sillyness is with the use of r8b to store the carry flag, then using
"add r8b, -1" to put the result back into carry.
Instead, the code should be something like this:
add rdx, rdi
mov rax, rdx
adc rcx, rsi
xor rax, rcx
ret
Naturally, for something this simple, I'd use unsigned __int128, but this came
up in large number math.
