https://bugs.llvm.org/show_bug.cgi?id=52298
Bug ID: 52298
Summary: [AArch64] Optimise xtn+xtn to uzp
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Keywords: beginner
Severity: enhancement
Priority: P
Component: Backend: AArch64
Assignee: unassignedb...@nondot.org
Reporter: pavel.il...@arm.com
CC: arnaud.degrandmai...@arm.com,
llvm-bugs@lists.llvm.org, smithp...@googlemail.com,
ties.st...@arm.com
Optimise truncate operations with a hi/lo pair into a single permute of half
the bit size of the input and just ignoring the top bits (which are truncated
out).
void d2 (short * restrict a, int *b, int n) {
for (int i = 0; i < n; i++)
a[i] = b[i];
}
clang -O3 generates
ldp q0, q1, [x9, #-16]
add x9, x9, #32
subs x8, x8, #8
xtn v0.4h, v0.4s
xtn v1.4h, v1.4s
stur d0, [x10, #-8]
str d1, [x10], #16
b.ne .LBB0_2
gcc -O3 combines the two `xtn`s into a `uzp`
ldp q0, q1, [x1]
add x1, x1, 32
uzp1 v0.8h, v0.8h, v1.8h
str q0, [x0], 16
cmp x1, x2
bne .L3
https://godbolt.org/z/dYozq6GPh
gcc commit
https://github.com/gcc-mirror/gcc/commit/52da40ffe2aaf086f622e513cc99a64bc7573a67#diff-e55f82ad6f42cf1df30ab3fffeba1fd00aea748d04e231dda6f247b5c7e1c35a
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