These are the the ones where we already had a Register object being used. Some places are still using unsigned which I did not convert.
This patch can now reland after 318c69de52b6 relanded #114827. This reverts commit 1683f84d289348ba6879635c4161979204f75230.
Maybe not needed but to avoid conflicts with #117307 Without revert of this one, but reverting #117307, the regenerated init-undef.mir became empty. This reverts commit be15fd5085680cc5ed9ec4f4f2258b504cdd55db.
When enabling subreg liveness tracking for AArch64, this pass fails because it tries to get the register class for the artificial subreg `sub_32_hi` of a 64-bit GPR. It tries to create an INIT_UNDEF instruction for the top 32-bits of the 64-bit GPR, which are not directly addressable, so getSubRegisterClass() returns a nullptr, crashing this pass. It should instead just avoid trying to create the INIT_UNDEF instruction.
InitUndef should also handle early-clobber / undef conflicts in inline asm operands. Do this by iterating over all_defs() instead of defs(). The newly added ARM test was generating an "unpredictable STXP instruction, status is also a source" error prior to this change. Fixes https://github.com/llvm/llvm-project/issues/106380.
The InitUndef pass works around a register allocation issue, where undef operands can be allocated to the same register as early-clobber result operands. This may lead to ISA constraint violations, where certain input and output registers are not allowed to overlap. Originally this pass was implemented for RISCV, and then extended to ARM in #77770. I've since removed the target-specific parts of the pass in #106744 and #107885. This PR reduces the pass to use a single requiresDisjointEarlyClobberAndUndef() target hook and enables it by default. The hook is disabled for AMDGPU, because overlapping early-clobber and undef operands are known to be safe for that target, and we get significant codegen diffs otherwise. The motivating case is the one in arm64-ldxr-stxr.ll, where we were previously incorrectly allocating a stxp input and output to the same register.
Multiple invocations of the pass could interfere with eachother, preventing some undefs being initialised. I found it very difficult to create a unit test for this due to it being dependent on particular allocations of a previous function. However, the bug can be observed here: https://godbolt.org/z/7xnMo41Gv with the creation of the illegal instruction `vnsrl.wi v9, v8, 0`
InitUndef currently always computes DeadLaneDetector, but only actually uses it if subreg liveness is enabled for the target. Make the calculation optional to avoid an unnecessary compile-time impact for targets that don't enable subreg liveness.
The InitUndef pass currently uses the getLargestSuperClass() hook (which is only used by that pass) to chose the register to initialize. This was done to reduce the number of undef init pseudos needed, e.g. so that the vrnov0 regclass would use the same pseudo as v0. After #106744 we use a single generic pseudo, so this is no longer necessary.