[PATCH] D77558: PowerPC: Don't hoist float multiply + add to fused operation on SPE

[Justin Hibbits via Phabricator via llvm-commits]

jhibbits added a comment.

In D77558#1966207 <https://reviews.llvm.org/D77558#1966207>, @shchenz wrote:

> >   This happens too late for the CTR usability test to veto using the CTR in a loop, and results in an assert "Invalid PPC CTR loop!".
>
> This means after not hoisting `fmul` makes a case have "Invalid PPC CTR loop!" assertion? This is a little surprised for me. Hoist or not hoist `fmul` should not impact CTR register.
>  Looking forward to your case.


Sorry, on a further reading of the summary I can see it can be a little confusing.  *hoisting* fmul + fadd to fma results in a library call, because SPE doesn't have a fma instruction.  Unfortunately, this transform is performed long after a loop is transformed to a CTR with bdnz loop, so it can't be caught at the loop transform time, and block the loop.  In addition to triggering that assert, hoisting two instructions into a function call is quite a pessimization anyway :)

I'll update the summary to clarify this.


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  https://reviews.llvm.org/D77558/new/

https://reviews.llvm.org/D77558