[PATCH] D120597: [RISCV] With Zbb, fold (sext_inreg (abs X)) -> (max X, (negw X))
Craig Topper via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Thu Mar 3 13:20:39 PST 2022
craig.topper added inline comments.
================
Comment at: llvm/lib/Target/RISCV/RISCVISelLowering.cpp:7541
+ cast<VTSDNode>(N->getOperand(1))->getVT() == MVT::i32 &&
+ DAG.ComputeNumSignBits(Src.getOperand(0)) > 32) {
+ SDLoc DL(N);
----------------
spatel wrote:
> spatel wrote:
> > craig.topper wrote:
> > > spatel wrote:
> > > > Can the `ComputeNumSignBits` be an assert rather than part of the predicate?
> > > The input is sign extended if the abs was promoted by type legalization, but I think it is possible to write (i64 (sext (i32 (trunc (i64 abs X)))) in the original IR and the input would not be sign extended.
> > Maybe I'm not understanding the pattern - is it possible to write a negative test?
> > If we sext_inreg from i32, does this model the transform:
> > https://alive2.llvm.org/ce/z/j4RdVa ?
> I'm still not seeing it after reading the comment/example:
> ashr X, 32 -> adds 32 signbits to at least 1 existing signbit
> How can this be under 33?
>
> https://alive2.llvm.org/ce/z/Rvk__m
>
Your shift result isn't being used your src function returned %abs not %ashr https://alive2.llvm.org/ce/z/eRHZww
This is the transform I'm trying to do here
```
define i64 @src(i64 %x) {
%abs = call i64 @llvm.abs.i64(i64 %x, i1 0)
%shl = shl i64 %abs, 32
%ashr = ashr i64 %shl, 32
ret i64 %ashr
}
define i64 @tgt(i64 %x) {
%f = freeze i64 %x
%negx = sub i64 0, %f
%shl = shl i64 %negx, 32
%ashr = ashr i64 %shl, 32
%max = call i64 @llvm.smax.i64(i64 %ashr, i64 %negx)
ret i64 %max
}
```
It's only valid if %x has 33 sign bits.
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D120597/new/
https://reviews.llvm.org/D120597
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