https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84547
Bug ID: 84547
Summary: Suboptimal code for masked shifts (ARM64)
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: nruslan_devel at yahoo dot com
Target Milestone: ---
Partially related to the Bug 84431 (see description of the problem there) but
observed on ARM64 instead of x86/x86-64. (Not sure about ARM32.)
Test example:
__uint128_t func(__uint128_t a, unsigned shift)
{
return a << (shift & 63);
}
aarch64-linux-gnu-gcc-7 -Wall -O2 -S test.c
GCC generates:
func:
and w2, w2, 63
mov w4, 63
sub w5, w4, w2
lsr x4, x0, 1
sub w3, w2, #64
lsl x1, x1, x2
cmp w3, 0
lsr x4, x4, x5
orr x1, x4, x1
lsl x4, x0, x3
lsl x0, x0, x2
csel x1, x4, x1, ge
csel x0, x0, xzr, lt
ret
While clang/llvm generates better code:
func: // @func
// BB#0:
and w8, w2, #0x3f
lsr x9, x0, #1
eor x11, x8, #0x3f
lsl x10, x1, x8
lsr x9, x9, x11
orr x1, x10, x9
lsl x0, x0, x8
ret
Another interesting case when __builtin_unreachable() is used:
__uint128_t func(__uint128_t a, unsigned shift)
{
if (shift > 63)
__builtin_unreachable();
return a << shift;
}
But in this case, neither clang/llvm, nor gcc seem to be able to optimize code
well.
