[llvm] [InstCombine] Extend folding of aggregate construction to cases when source aggregates are partially available (PR #100828)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

I don't think we can benefit from introducing more aggregate phi nodes. If these phi nodes are converted into selects in subsequent passes (e.g., SimplifyCFG), it is hard for InstCombine to fold. See https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1351#discussion_r1802502189.

If you think this patch will reduce total number of instructions, I don't agree with this argument. Can you provide some performance data on SPEC/llvm-test-suite?

Compilation time impact looks ok. Don't worry about this :)

I will accept this patch if:
1. most of the regressions related to `selects on aggregate types` are fixed.
2. This patch results in a net performance improvement on some benchmarks.


https://github.com/llvm/llvm-project/pull/100828