https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68110
Bug ID: 68110
Summary: __builtin_sub_overflow unsigned performance issue
Product: gcc
Version: 5.2.1
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: eggert at gnu dot org
Target Milestone: ---
I ran into this minor performance issue when changing Gnulib's lib/intprops.h
to use the new __builtin_sub_overflow function. I found that
__builtin_sub_overflow is less efficient than the portable C code that it
replaced, when the operands and result are unsigned, or unsigned long, or
unsigned long long. To reproduce the problem, compile and run the following
program with 'gcc -O2 -S' on x86-64:
_Bool f1 (unsigned long long a, unsigned long long b)
{
return a < b;
}
_Bool f2 (unsigned long long a, unsigned long long b)
{
unsigned long long r;
return __builtin_sub_overflow (a, b, &r);
}
Although the functions are semantically equivalent, f1 uses only 3
instructions:
f1:
cmpq %rsi, %rdi
setb %al
ret
whereas f2 uses 5 instructions:
f2:
movq %rdi, %rax
subq %rsi, %rax
cmpq %rdi, %rax
seta %al
ret
There is a similar problem for x86.
