pmatos added a comment.
Proposal to fix https://github.com/llvm/llvm-project/issues/62703
This not ready to land yet but it should open a discussion on if this is a good
fix for this issue.
If we want to go this route we need: support for 64bits, tests.
A quick summary of why this is needed. The function :
inline u32 bswap(u32 x) {
return __builtin_rotateleft32((x & 0xFF00FF00), 8)
| __builtin_rotateright32((x & 0x00FF00FF), 8);
}
generates really poor code in WebAssembly. A rotate should be generated but
instead we get quite a large amount of code just to set up constants and
perform a shift. The issue is that the rotateleft is lowered to a fshl(%and,
%and, 8). During instcombine, the second argument is simplified since %and is
the result of a bitwise operation. However, this results in the wasm backend
not being able to generate the rotate since the first and second arguments to
the fshl are not any longer the same. The generated code is:
bswap: # @bswap
.functype bswap (i32) -> (i32)
# %bb.0: # %entry
local.get 0
i32.const 24
i32.shl
local.get 0
i32.const 65280
i32.and
i32.const 8
i32.shl
i32.or
local.get 0
i32.const 8
i32.shr_u
i32.const 65280
i32.and
local.get 0
i32.const 24
i32.shr_u
i32.or
i32.or
# fallthrough-return
end_function
With the fix this becomes:
bswap: # @bswap
.functype bswap (i32) -> (i32)
# %bb.0: # %entry
local.get 0
i32.const 16711935
i32.and
i32.const 8
i32.rotr
local.get 0
i32.const -16711936
i32.and
i32.const 8
i32.rotl
i32.or
# fallthrough-return
end_function
The alternative way to implement something like this would be to block the
instcombine on fshl/fshr instructions for the wasm backend, however adding
target dependent stuff to instcombine feels even more icky than this patch. I
welcome suggestions on how to improve the patch, since the hack in `emitRotate`
is also not great, and I have not seen other places lowering a generic builtin
into a target specific intrinsic.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D150670/new/
https://reviews.llvm.org/D150670
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