forked from OSchip/llvm-project
3b572002a2
Summary: SI_ELSE is lowered into two parts: s_or_saveexec_b64 dst, src (at the start of the basic block) s_xor_b64 exec, exec, dst (at the end of the basic block) The idea is that dst contains the exec mask of the preceding IF block. It can happen that SIWholeQuadMode decides to switch from WQM to Exact mode inside the basic block that contains SI_ELSE, in which case it introduces an instruction s_and_b64 exec, exec, s[...] which masks out bits that can correspond to both the IF and the ELSE paths. So the resulting sequence must be: s_or_savexec_b64 dst, src s_and_b64 exec, exec, s[...] <-- added by SIWholeQuadMode s_and_b64 dst, dst, exec <-- added by SILowerControlFlow s_xor_b64 exec, exec, dst Whether to add the additional s_and_b64 dst, dst, exec is currently determined via the ExecModified tracking. With this change, it is instead determined by an additional flag on SI_ELSE which is set by SIWholeQuadMode. Finally: It also occured to me that an alternative approach for the long run is for SILowerControlFlow to unconditionally emit s_or_saveexec_b64 dst, src ... s_and_b64 dst, dst, exec s_xor_b64 exec, exec, dst and have a pass that detects and cleans up the "redundant AND with exec" pattern where possible. This could be useful anyway, because we also add instructions s_and_b64 vcc, exec, vcc before s_cbranch_scc (in moveToALU), and those are often redundant. I have some pending changes to how KILL is lowered that could also benefit from such a cleanup pass. In any case, this current patch could help in the short term with the whole ExecModified business. Reviewers: tstellarAMD, arsenm Subscribers: arsenm, llvm-commits, kzhuravl Differential Revision: https://reviews.llvm.org/D22846 llvm-svn: 276972 |
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