This patch enables folding following instructions under -ffast-math flag: log10(pow(10.0,x)) -> x, log2(pow(2.0,x)) -> x

# Details

# Diff Detail

### Event Timeline

Seems ok.

lib/Analysis/InstructionSimplify.cpp | ||
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4480 | Hmm, do we want to canonicalize |

lib/Analysis/InstructionSimplify.cpp | ||
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4480 | If |

lib/Analysis/InstructionSimplify.cpp | ||
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4480 | It's a specialized form of the more general math function, so we can assume it has a faster implementation. I think we should add that fold and reduce this patch. |

lib/Analysis/InstructionSimplify.cpp | ||
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4480 | There is a counter-argument to that: (that i have thought of after asking, but did not post) I.e. is there some situation where we might end up with |

lib/Analysis/InstructionSimplify.cpp | ||
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4480 | Instsimplify may be used independently of instcombine by other passes, so yes, it is possible that the non-canonical form will be seen here. It's not generally a concern with the typical optimization pipeline since we run instcombine...a lot. So the full solution is: leave this patch as-is (handle both forms) and add the instcombine too. |

LG unless others have more comments (@spatel ?)

You probably know this already, but there are some other low-hanging fruit here, e.g.

https://godbolt.org/z/Trdf_e

https://www.wolframalpha.com/input/?i=log2(x)+%2F+log2(10)+%3D+log10(x)

Thanks, I'll dive into it. Btw, `pow(2.0, x)` --> `exp2(x)` is already implemented, see https://godbolt.org/z/UADcWe.

If you're interested in math call transforms, you might also want to look at:

https://bugs.llvm.org/show_bug.cgi?id=40541

One more comment: 'reassoc' is generally enough to go crazy with FP, but we might also want to require 'nsz' on this and related transforms to be safer because:

x = -0.0

exp2(x) = 1.0

log2(exp2(x)) = 0.0 <-- output flipped sign from input