https://bugs.llvm.org/show_bug.cgi?id=43197
Bug ID: 43197
Summary: Signed division with power of two can be faster
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: david.bolvan...@gmail.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
int foo(int i) {
return i/(1 << 4);
}
foo(int): # @foo(int)
mov eax, edi
sar eax, 31
shr eax, 28
add eax, edi
sar eax, 4
ret
Dispatch Width: 6
uOps Per Cycle: 3.81
IPC: 2.86
Block RThroughput: 1.5
GCC:
foo(int):
test edi, edi
lea eax, [rdi+15]
cmovns eax, edi
sar eax, 4
ret
Dispatch Width: 6
uOps Per Cycle: 4.38
IPC: 3.13
Block RThroughput: 1.2
GCC's perf is better, microbenchmark confirms it:
int main(void) {
int s = 0;
for (int i = -60000000; i < 1000000000; ++i)
s+= i/(1 << 4);
return s;
}
Compiled with -O1:
clang -O1 -march=native lop.c
time ./a.out
real 0m0,658s
user 0m0,655s
sys 0m0,004s
gcc-9 -O1 -march=native lop.c
time ./a.out
real 0m0,605s
user 0m0,605s
sys 0m0,000s
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