https://bugs.llvm.org/show_bug.cgi?id=44126
Bug ID: 44126
Summary: Improve IR for code which finds position of highest
bit
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]
https://github.com/facebook/zstd/blob/47034cd6c31125fdba3155abe9a618f580b4f3eb/programs/fileio.c#L1789
unsigned long long FIO_highbit64(unsigned long long v)
{
unsigned count = 0;
v >>= 1;
while (v) { v >>= 1; count++; }
return count;
}
should be same as:
unsigned long long FIO_highbit64a(unsigned long long v)
{
return 63 - __builtin_clzll(v);
}
But first version has worse IR and codegen:
define dso_local i64 @_Z13FIO_highbit64y(i64 %0) local_unnamed_addr #0 {
%2 = lshr i64 %0, 1
%3 = call i64 @llvm.ctlz.i64(i64 %2, i1 false), !range !2
%4 = sub nuw nsw i64 64, %3
ret i64 %4
}
=>
define dso_local i64 @_Z14FIO_highbit64ay(i64 %0) local_unnamed_addr #1 {
%2 = tail call i64 @llvm.ctlz.i64(i64 %0, i1 true), !range !3
%3 = xor i64 %2, 63
ret i64 %3
}
It would be good to not forget on trunc variant:
unsigned FIO_highbit64(unsigned long long v)
{
unsigned count = 0;
v >>= 1;
while (v) { v >>= 1; count++; }
return count;
}
define dso_local i32 @_Z13FIO_highbit64y(i64 %0) local_unnamed_addr #0 {
%2 = lshr i64 %0, 1
%3 = call i64 @llvm.ctlz.i64(i64 %2, i1 false), !range !2
%4 = trunc i64 %3 to i32
%5 = sub nsw i32 64, %4
ret i32 %5
}
=>
define dso_local i32 @_Z13FIO_highbit64y(i64 %0) local_unnamed_addr #0 {
%2 = tail call i64 @llvm.cttz.i64(i64 %0, i1 true), !range !2
%3 = trunc i64 %2 to i32
%4 = xor i32 %3, 63
ret i32 %4
}
FIO_highbit64(unsigned long long):
shr rdi
je .LBB0_1
bsr rcx, rdi
xor rcx, 63
mov eax, 64
sub eax, ecx
ret
.LBB0_1:
mov ecx, 64
mov eax, 64
sub eax, ecx
ret
vs:
FIO_highbit64(unsigned long long):
bsf rax, rdi
xor eax, 63
ret
--
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