https://bugs.llvm.org/show_bug.cgi?id=48970
Bug ID: 48970
Summary: Unaligned read followed by bswap generates suboptimal
code
Product: libraries
Version: 11.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: RISC-V
Assignee: unassignedb...@nondot.org
Reporter: eduardosanchezmu...@gmail.com
CC: a...@lowrisc.org, llvm-bugs@lists.llvm.org
On RISC-V, an unaligned read followed by a bswap produces suboptimal code.
Given the following IR:
declare i16 @llvm.bswap.i16(i16)
define i16 @read(i16* %p) {
start:
%v = load i16, i16* %p, align 1
ret i16 %v
}
define i16 @read_swap(i16* %p) {
start:
%v = load i16, i16* %p, align 1
%v2 = tail call i16 @llvm.bswap.i16(i16 %v)
ret i16 %v2
}
compiled with llc -mtriple=riscv64-unknown-linux-gnu -O3
it produces the following assembly:
read:
lb a1, 1(a0)
lbu a0, 0(a0)
slli a1, a1, 8
or a0, a0, a1
ret
read_swap:
lb a1, 1(a0)
lbu a0, 0(a0)
slli a1, a1, 8
or a0, a0, a1
slli a1, a0, 40
addi a2, zero, 255
slli a2, a2, 48
and a1, a1, a2
slli a0, a0, 56
or a0, a0, a1
srli a0, a0, 48
ret
The code for read is generated as expected. However, the code for read_swap can
be simplified to:
read_swap:
lb a1, 0(a0)
lbu a0, 1(a0)
slli a1, a1, 8
or a0, a0, a1
ret
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