https://bugs.llvm.org/show_bug.cgi?id=37421
Bug ID: 37421
Summary: [AArch64] Generated code for rotate-lefts has an
unnecessary extra and
Product: libraries
Version: 6.0
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: AArch64
Assignee: unassignedb...@nondot.org
Reporter: fabi...@radgametools.com
CC: llvm-bugs@lists.llvm.org
void rotateInLoop2(unsigned int *arr, const bool *control, int count, int rot0,
int rot1)
{
for (int i = 0; i < count; ++i)
{
int rot = control[i] ? rot1 : rot0;
arr[i] = (arr[i] << (rot & 31)) | (arr[i] >> (-rot & 31));
}
}
clang 6.0 "clang -O2 -fno-tree-vectorize -target aarch64-linux-android"
produces for the main loop:
.LBB1_2: // =>This Inner Loop Header: Depth=1
ldrb w9, [x1], #1
ldr w10, [x0]
cmp w9, #0 // =0
csel w9, w3, w4, eq
neg w9, w9
and w9, w9, #0x1f // <-- this is unnecessary
ror w9, w10, w9
subs x8, x8, #1 // =1
str w9, [x0], #4
b.ne .LBB1_2
Probably related to https://bugs.llvm.org/show_bug.cgi?id=27582
The conditional select for the "rot" is to work around the issue with shift
counts hoisted outside the loop mentioned in
https://bugs.llvm.org/show_bug.cgi?id=37387
-fno-tree-vectorize because with vectorization on, the loop gets unrolled 2x
(presumably to use ldp/stp?)
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