stage2: Teach Liveness that some safety checks do not observably modify memory#13233
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andrewrk merged 1 commit intoziglang:masterfrom Dec 12, 2022
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stage2: Teach Liveness that some safety checks do not observably modify memory#13233andrewrk merged 1 commit intoziglang:masterfrom
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This change adds to Liveness a simple pattern match for the try-like `.condbr` blocks emitted by Sema's safety checks. This allows us to determine that these do not modify memory, which permits us to elide additional loads in the backend. As @Vexu points out in the main issue, this is probably not a complete solution on its own. We'll still want a way to reliably narrow the load/copy when performing several consecutive accesses, such as `foo.arr[x][y].z` Resolves ziglang#12215
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Spun off from #13074
This is a crude pattern match for try-like
.cond_brblocks emitted by Sema's safety checks. It looks for:This matches the signature of many of our safety checks which look like this:
The reasoning here is that control flow either diverges (the call must be
noreturnto be at the end of the block) or continues directly to the following instructions in the block without modifying memory. This was inspired by #11772 (comment)Resolves #12215