llvm-project

76e14deb - [X86][BreakFalseDeps] Using reverse order for undef register selection (#137569)

[X86][BreakFalseDeps] Using reverse order for undef register selection (#137569) BreakFalseDeps picks the best register for undef operands if instructions have false dependency. The problem is if the instruction is close to the beginning of the function, ReachingDefAnalysis is over optimism to the unused registers, which results in collision with registers just defined in the caller. This patch changes the selection of undef register in an reverse order, which reduces the probability of register collisions between caller and callee. It brings improvement in some of our internal benchmarks with negligible effect on other benchmarks.