[PATCH] D38085: Use the basic cost if a GEP is not used as addressing mode

[Chandler Carruth via Phabricator via llvm-commits]

chandlerc added a comment.

This patch got reverted again, and I wanted to provide some insight as to why...

We saw a pretty dramatic and immediate regression in benchmarks due to failed inlining. One of the benchmarks is open source (Gipfeli) but it can be hard or impossible to reproduce the benchmark rigging that shows the clear issue. But looking at just one benchmark doesn't seem very informative...

The issue is that we saw significant increases in inline cost. This very likely isn't just in one place but is true across the board, and I think I can explain some of why....

I think the real problem here is that we simply can't determine "what will fold" into an addressing mode this early in the pipeline. Looking at users I think is the wrong model, because those users are extremely likely to change as a result of inlining and the rest of the optimization pipeline. While it may be somewhat counter-intuitive that we get better results by *assuming* that a GEP will fold at some point into an addressing mode, currently that seems to be a better assumption than that the users seen when querying the cost are actually the correct users to analyze.


https://reviews.llvm.org/D38085