[PATCH] fold: sqrt(x * x * y) -> fabs(x) * sqrt(y)

[Sanjay Patel]

================
Comment at: lib/Transforms/Utils/SimplifyLibCalls.cpp:1270
@@ +1269,3 @@
+  if (Instruction *I = dyn_cast<Instruction>(Op)) {
+    if (I->getOpcode() == Instruction::FMul && I->hasUnsafeAlgebra()) {
+      // We're looking for a repeated factor in a multiplication tree,
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hfinkel wrote:
> Hrmm... is this right, or do we need to check the function attribute here? I'm not sure that the argument having the unsafe-algebra flag means that we can change its use in a non-strict way.
I think this goes back to the discussion of whether 'fast' is infectious in http://reviews.llvm.org/D5584. :)
In that case, we optimized away a sqrt call even though it wasn't explicitly marked 'fast'. This is a similar transform. We've just reversed the order of the fmul and sqrt.

I thought I saw some other precedence for just using the instruction-level flags, but I'm not finding it now. There's an optimization for log2() in InstCombineMulDivRem that checks whether the log2 intrinsic hasUnsafeAlgebra...I don't know how that is even specified in IR. I thought the IR fast-math flags only apply to fmul, fdiv, fadd, fsub, and frem? At least, that's what the LangRef says. FWIW, that log2 optimization doesn't appear to ever trigger, and I don't see any test case for it (r169025).

http://reviews.llvm.org/D5787