https://bugs.llvm.org/show_bug.cgi?id=51563
Bug ID: 51563
Summary: incorrect conversion of sqrdmulh+sqsub to sqrdmlsh
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: jz...@google.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk, sam.par...@arm.com,
sjoerd.mei...@arm.com
Created attachment 25180
--> https://bugs.llvm.org/attachment.cgi?id=25180&action=edit
repro
The sequence of sqrdmulh followed by sqsub is being converted to sqrdmlsh which
shifts the rounding from the first multiply to the result causing an off by one
in some cases. gcc (tested with aarch64-linux-gnu-gcc (Debian 10.2.1-6+build2)
10.2.1 20210110) does not perform this conversion.
Reproduced with llvmorg-14-init-1836-ge4888be74e34 and releases going back to
3.9.1.
# repros with -O2/-O3, other levels aside from 0 untested.
$ clang -march=armv8.1-a -target aarch64-pc-linux-gnu -O2 \
sqrdmulh_sqsub-sqrdmlsh-repro.c
See also:
https://godbolt.org/z/zhKnMT15s
https://chromium.googlesource.com/codecs/libgav1/+/65bb0dae17596e88ec33cba5d53d1ce54e2fcfcf
non_opt: // @non_opt
sub sp, sp, #64 // =64
movi v0.8h, #1, lsl #8
str q0, [sp, #48]
movi v0.8h, #1
str q0, [sp, #32]
ldr q0, [sp, #48]
mov w8, #42944
dup v1.8h, w8
sqrdmulh v0.8h, v0.8h, v1.8h
str q0, [sp, #16]
ldr q0, [sp, #32]
ldr q1, [sp, #16]
sqsub v0.8h, v0.8h, v1.8h
str q0, [sp]
ldr q0, [sp]
add sp, sp, #64 // =64
ret
opt: // @opt
mov w8, #42944
movi v1.8h, #1, lsl #8
dup v2.8h, w8
movi v0.8h, #1
sqrdmlsh v0.8h, v1.8h, v2.8h
ret
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