[PATCH] D104247: [DAGCombine] reassoc flag shouldn't enable contract

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:13258
+    bool CanFuse = Options.UnsafeFPMath || N->getFlags().hasAllowContract();
     // fold (fsub (fma x, y, (fmul u, v)), z)
     //   -> (fma x, y (fma u, v, (fneg z)))
----------------
jsji wrote:
> mcberg2017 wrote:
> > Some of these patterns below have operands/operations which change order, they should be audited and checked reassoc FMF to enable.  Perhaps a follow on change?
> Good point, sure, will follow up.
Sorry, I missed this difference. The patch as-is is allowing miscompiles IIUC. For example, this test of the pattern:

```
; x*y + u*v - z
define float @f(float %x, float %y, float %z, float %u, float %v) {
  %xy = fmul contract float %x, %y
  %uv = fmul contract float %u, %v
  %xyuv = fadd contract float %xy, %uv
  %xyuvz = fsub contract float %xyuv, %z
  ret float %xyuvz
}

```

We are not allowed to fuse the trailing fsub into fma based on what we decided in D89527, but that's what happens now with:

```
$ llc -o - fma.ll -mtriple=powerpc64le -ppc-asm-full-reg-names
	xsmsubasp f3, f4, f5 ; u*v - z
	xsmaddasp f3, f1, f2 ; x*y + (u*v - z)
	fmr f1, f3
	blr

```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104247/new/

https://reviews.llvm.org/D104247