https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97961
Bug ID: 97961
Summary: unnecessary moves with __builtin_{add,sub}_overflow_p
and __int128
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: denis.campredon at gmail dot com
Target Milestone: ---
In #97950 Jackub told me to open a new bug for that.
The snippet bellow has the following problems
- f1 and f2 generate 4 unnecessary moves
mov r9, rdi
mov r8, rsi
mov rsi, r9
mov rdi, r8
- f4 has "only" 2 unnecessary moves
mov r9, rdi
mov rdi, rsi
- f3 should be identical to f4 except for the flag checking.
------------
bool f1(unsigned __int128 a,unsigned __int128 b) {
return __builtin_add_overflow_p(a, b, (unsigned __int128)0);
}
bool f2(__int128 a,__int128 b) {
return __builtin_add_overflow_p(a, b, (__int128)0);
}
bool f3(unsigned __int128 a,unsigned __int128 b) {
return __builtin_sub_overflow_p(a, b, (unsigned __int128)0);
}
bool f4(__int128 a,__int128 b) {
return __builtin_sub_overflow_p(a, b, (__int128)0);
}
------------
asm generated
------------
f1(unsigned __int128, unsigned __int128):
mov r9, rdi
mov r8, rsi
mov rsi, r9
mov rdi, r8
add rsi, rdx
adc rdi, rcx
setc al
ret
f2(__int128, __int128):
mov r9, rdi
mov r8, rsi
mov rsi, r9
mov rdi, r8
add rsi, rdx
adc rdi, rcx
seto al
ret
f3(unsigned __int128, unsigned __int128):
mov r9, rdi
mov r8, rsi
mov rdi, r8
mov rax, r9
mov r8, rdx
sub rax, r8
mov rdx, rdi
sbb rdx, rcx
cmp r9, rax
mov rcx, rdi
sbb rcx, rdx
setc al
ret
f4(__int128, __int128):
mov r9, rdi
mov rdi, rsi
cmp r9, rdx
sbb rdi, rcx
seto al
ret
-------------------
