https://bugs.llvm.org/show_bug.cgi?id=48640
Bug ID: 48640
Summary: Missed optimization for ((X ashr C1) & C2) == 0
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
void foo();
void bad1(int e)
{
if (((e >> 31) & 64) == 0) foo();
}
void bad2(int e)
{
if (((e >> 31) & 64) != 0) foo();
}
void ok1(unsigned e)
{
if (((e >> 31) & 64) == 0) foo();
}
void ok2(unsigned e)
{
if (((e >> 31) & 64) != 0) foo();
}
LLVM:
bad1(int): # @bad1(int)
shr edi, 25
test dil, 64
jne .LBB0_1
jmp foo() # TAILCALL
.LBB0_1:
ret
bad2(int): # @bad2(int)
shr edi, 25
test dil, 64
jne .LBB1_2
ret
.LBB1_2:
jmp foo() # TAILCALL
ok1(unsigned int): # @ok1(unsigned int)
jmp foo() # TAILCALL
ok2(unsigned int): # @ok2(unsigned int)
ret
GCC:
bad1(int):
test edi, edi
jns .L4
ret
.L4:
jmp foo()
bad2(int):
test edi, edi
js .L7
ret
.L7:
jmp foo()
ok1(unsigned int):
jmp foo()
ok2(unsigned int):
ret
https://godbolt.org/z/x85bf6
So..
((X ashr C1) & C2) != 0 to X < 0
..and
((X ashr C1) & C2) == 0 to X >= 0?
define i1 @src(i32 %0) {
%1:
%2 = ashr i32 %0, 31
%3 = and i32 %2, 64
%4 = icmp eq i32 %3, 0
ret i1 %4
}
=>
define i1 @tgt(i32 %0) {
%1:
%r = icmp sge i32 %0, 0
ret i1 %r
}
Transformation seems to be correct!
define i1 @src(i32 %0) {
%1:
%2 = ashr i32 %0, 31
%3 = and i32 %2, 64
%4 = icmp ne i32 %3, 0
ret i1 %4
}
=>
define i1 @tgt(i32 %0) {
%1:
%r = icmp slt i32 %0, 0
ret i1 %r
}
Transformation seems to be correct!
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
