bc7d88faf1
This is a conservative workaround for broken liveness tracking of SUBREG_TO_REG to speculatively fix all targets. The current reported failures are on X86 only, but this issue should appear for all targets that use SUBREG_TO_REG. The next minimally correct refinement would be to disallow only implicit defs. The coalescer now introduces implicit-defs of the super register to track the dependency on other subregisters. If we see such an implicit operand, we cannot simply treat the subregister def as the result operand in case downstream users depend on the implicitly defined parts. Really target implementations should be considering the implicit defs and trying to interpret them appropriately (maybe with some generic helpers). The full implicit def could possibly be reported as the move result, rather than the subregister def but that requires additional work. Hopefully fixes #64060 as well. This needs to be applied to the release branch. https://reviews.llvm.org/D156346
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