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optimize pow(x, i1) / pow(x, i2) with fast-math #67216

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k-arrows opened this issue Sep 23, 2023 · 0 comments
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optimize pow(x, i1) / pow(x, i2) with fast-math #67216

k-arrows opened this issue Sep 23, 2023 · 0 comments
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llvm:instcombine missed-optimization

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@k-arrows
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Consider the following:

#include <cmath>

double f1(double x)
{
    return pow(x, 2) / pow(x, 1);
}

double f2(double x)
{
    return pow(x, 3) / pow(x, 1);
}

https://godbolt.org/z/jjKE5n8o1
If -Ofast is specified, GCC just calculates x * x for f2, but Clang performs floating-point division.

When I lower the version to Clang 14.0.0, it behaves as expected, has anything changed?
@vfdff vfdff mentioned this issue Sep 23, 2023
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vfdff added a commit to vfdff/llvm-project that referenced this issue Oct 21, 2023
@vfdff
[InstCombine] optimize powi(X,Y)/X with Ofast
fd73838 
Try to transform the powi(X, Y) / X into powi(X, Y-1) with Ofast.
For this case, when the Y is 3, then powi(X, 2) is replaced by X * X in
the further step.

Fixes llvm#67216
@vfdff vfdff closed this as completed in 7e3d110 Oct 21, 2023
@k-arrows k-arrows mentioned this issue Jan 6, 2024
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[InstCombine] optimize powi(X,Y)/X with Ofast vfdff/llvm-project
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@EugeneZelenko @k-arrows