[PATCH] D74436: Change clang option -ffp-model=precise to select ffp-contract=on

[Haowei Wu via Phabricator via cfe-commits]

haowei added a comment.

Can we revert this patch please? If I understood correctly, this patch changed the default behavior of clang when generating code for float point arithmetic. After this patch `-ffp-contract` was set to on if this flag was not specified. In theory this makes clang use FMA instructions that should be slightly more efficient, but there are cases in the field that certain programs relies on the stable rounding behavior of float point arithmetic, and this patch broke that. And in fact we already seen breakages in Fuchsia, Chromium and SPEC benchmark suite and there might be other people seeing the same breakages but still working on bisecting this breaking change.

Even if this breaking change is reasonable to proceed and receive condense from major clang users, this patch lacks enough test coverage for codegen of float point arithmetic on major CPU platforms without related flag specified. Nobody will discover this is a default behavior breaking patch by just reading this patch. This should be reverted and reworked.


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  https://reviews.llvm.org/D74436/new/

https://reviews.llvm.org/D74436