https://bugs.llvm.org/show_bug.cgi?id=34924
Bug ID: 34924
Summary: Remove 'zero shift' guards from rotation patterns
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
For rotation patterns we often have code that prevents out of bounds shifts.
Once we have created rotations its no longer necessary for these guards and
they should be removed to prevent pointless branches.
unsigned rotl(unsigned a, unsigned b) {
return (b == 0 ? a : ((a >> (32-b)) | (a << b)));
}
unsigned rotr(unsigned a, unsigned b) {
return (b == 0 ? a : ((a << (32-b)) | (a >> b)));
}
define i32 @rotl(i32, i32) {
%3 = icmp eq i32 %1, 0
br i1 %3, label %9, label %4
; <label>:4: ; preds = %2
%5 = sub i32 32, %1
%6 = lshr i32 %0, %5
%7 = shl i32 %0, %1
%8 = or i32 %6, %7
br label %9
; <label>:9: ; preds = %2, %4
%10 = phi i32 [ %8, %4 ], [ %0, %2 ]
ret i32 %10
}
define i32 @rotr(i32, i32) {
%3 = icmp eq i32 %1, 0
br i1 %3, label %9, label %4
; <label>:4: ; preds = %2
%5 = sub i32 32, %1
%6 = shl i32 %0, %5
%7 = lshr i32 %0, %1
%8 = or i32 %6, %7
br label %9
; <label>:9: ; preds = %2, %4
%10 = phi i32 [ %8, %4 ], [ %0, %2 ]
ret i32 %10
}
llc -mtriple=x86_64-unknown -mcpu=btver2
rotl:
testl %esi, %esi
je .LBB0_2
movl %esi, %ecx
roll %cl, %edi
.LBB0_2:
movl %edi, %eax
retq
rotr:
testl %esi, %esi
je .LBB1_2
movl %esi, %ecx
rorl %cl, %edi
.LBB1_2:
movl %edi, %eax
retq
--
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