https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99620
Bug ID: 99620
Summary: Subtract with borrow (SBB) missed optimization
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: chfast at gmail dot com
Target Milestone: ---
Hi.
For the 128-bit precision subtraction: SUB + SBB the optimization depends on
the how the carry bit condition is specified in the code. In the first case
below everything works nicely, but in the second we have unnecessary CMP in the
final code.
I believe the second carry bit condition is simpler (does not require unsigned
integer wrapping behavior) and does not have dependency on the first
subtraction.
using u64 = unsigned long;
struct u128
{
u64 l;
u64 h;
};
auto sub_good(u128 a, u128 b)
{
auto l = a.l - b.l;
auto k = l > a.l;
auto h = a.h - b.h - k;
return u128{l, h};
}
auto sub_bad(u128 a, u128 b)
{
auto l = a.l - b.l;
auto k = a.l < b.l;
auto h = a.h - b.h - k;
return u128{l, h};
}
sub_good(u128, u128):
mov rax, rdi
sub rax, rdx
sbb rsi, rcx
mov rdx, rsi
ret
sub_bad(u128, u128):
cmp rdi, rdx
mov rax, rdi
sbb rsi, rcx
sub rax, rdx
mov rdx, rsi
ret
If you think this is easy to fix, I would like to give it a try if I could get
some pointers where to start.
