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133654
2017 Jun 06
2
Change undef to poison in a few operations
Hi,
Lately we have come to realize how undef makes our life complicated..
Therefore in this email we propose to change the behavior of a few
instruction to yield poison instead of undef in error cases. This follows
the suggestion of Eli in https://reviews.llvm.org/D33654.
Why is undef so bad?
- I believe it's not possible to make newgvn correct with undef. See for
example the discussion here: https://bugs.llvm.org/show_bug.cgi?id=33165
- A bunch of optimizations are correct with poison but not with undef (see
John's blog posts, my talk at LLVM conf last ye...