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CodeGenPrepare knows how to move a zext of a load into the same basic block where the load lives. The goal is to help ISel match a zero-extending load instead of two separated instructions. CGP attempts to move a zext computation even if it lives in a basic block that does not post-dominate the load's basic block. That means, the hoisted zext may be speculated. Preserving the zext location would hurt the debugging experience and the quality of sample pgo. With this patch, when moving a zext near to its associated load, CGP no longer propagates the zext's debug location. Instead, CGP conservatively reuses the same debug location for the load and the zext. An alternative approach would be to assign an artificial line-0 location to the zext. However we don't want to over-use the 'line-0' for this particular case because it would have a size cost in the line-table section for no additional benefit. Differential Revision: https://reviews.llvm.org/D25611 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284377 91177308-0d34-0410-b5e6-96231b3b80d8
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