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This patch resolves the suboptimal codegen described in http://llvm.org/pr47873 . When CodeGenPrepare lowers select into a conditional branch, a freeze instruction is inserted. It is then translated to `BRCOND(FREEZE(SETCC))` in SelDag. The `FREEZE` in the middle of `SETCC` and `BRCOND` was causing a suboptimal code generation however. This patch adds `BRCOND(FREEZE(cond))` -> `BRCOND(cond)` fold to DAGCombiner to remove the `FREEZE`. To make this optimization sound, `BRCOND(UNDEF)` simply should nondeterministically jump to the branch or not, rather than raising UB. It wasn't clear what happens when the condition was undef according to the comments in ISDOpcodes.h, however. I updated the comments of `BRCOND` to make it explicit (as well as `BR_CC`, which is also a conditional branch instruction). Note that it diverges from the semantics of `br` instruction in IR, which is explicitly UB. Since the UB semantics was necessary to explain optimizations that use branching conditions, and SelDag doesn't seem to have such optimization, I think this divergence is okay. Reviewed By: spatel Differential Revision: https://reviews.llvm.org/D92015 |
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