Loop canonicalization should always create loop pre-headers #77033
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BruceForstall
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area-CodeGen-coreclr
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch Issue DetailsIt would simplify a lot of code if loop canonicalization always creates pre-header blocks, and they are maintained late into compilation. This would require both allowing them to exist, but also to be maintained, e.g., #62665.
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This phase was recently removed with #83310 |
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As part of finding natural loops and creating the loop table, create a loop pre-header for every loop. This simplifies a lot of downstream phases, as the loop pre-header will be guaranteed to exist, and will already exist in the dominator tree. Introduce code to preserve an empty pre-header block through the optimization phases. Remove now unnecessary code in hoisting and elsewhere. Fixes dotnet#77033, dotnet#62665
* Always create loop pre-header As part of finding natural loops and creating the loop table, create a loop pre-header for every loop. This simplifies a lot of downstream phases, as the loop pre-header will be guaranteed to exist, and will already exist in the dominator tree. Introduce code to preserve an empty pre-header block through the optimization phases. Remove now unnecessary code in hoisting and elsewhere. Fixes #77033, #62665 * Fix loop unrolling to work with loop pre-headers * Add `optLoopsRequirePreHeaders` variable * Prevent removing pre-header blocks * Allow removing unreachable pre-headers Disallow creating pre-header after SSA is built * Make optLoopCloningEnabled() static * Teach loop cloning to expect and respect loop pre-headers * Remove special case pre-header handling in hoisting * Remove unused SSA update code in fgCreateLoopPreHeader * Remove unneeded pre-header code from fgDominate * Remove workaround to avoid extraneous LSRA diffs due to bbNum ordering * Update comments * Improve loop table rebuilding with pre-headers When the loop table is built, it looks around for various loop patterns, including looking for a guaranteed-executed, pre-loop constant initializer. This is used in loop cloning and loop unrolling. It needs to look "a little harder" in the case we created loop pre-headers, then rebuild the loop table (currently, only due to loop unrolling of loops that contain nested loops). The new code only allows for empty pre-headers. This works since in our current phase ordering, no hoisting happens by the time the loop table is rebuilt. (Actually, it's currently not necessary to do this at all, since the constant initializer info is only used by cloning and loop unrolling, both of which have finished by the time the loop table is rebuilt. However, we might someday choose to rebuild the loop table after cloning and before unrolling, at which point it would be necessary.) * Update comments
ghost
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It would simplify a lot of code if loop canonicalization always creates pre-header blocks, and they are maintained late into compilation. This would require both allowing them to exist, but also to be maintained, e.g., #62665.
Another motivating issue: #68844 (comment)
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