aykevl added a comment. Apparently there is also another patch that tries to do something very similar: D81223 <https://reviews.llvm.org/D81223>.
In D72404#3305275 <https://reviews.llvm.org/D72404#3305275>, @mehdi_amini wrote: > Why can't you build the object files with `-Os` in the first place instead of > changing the optimization level between the compilation and the linking > phases? I'm not changing the optimization level. The bitcode files are built with an equivalent of `-Oz` and have the `optsize` and `minsize` attributes. It's the optimization passes in the linker (ThinLTO) that don't respect these attributes. In D72404#3305267 <https://reviews.llvm.org/D72404#3305267>, @steven_wu wrote: > Before I say yes or no to this patch, have you figured out what are the > passes that causes the most size regression? Ideally, with function > attributes on the function, it shouldn't be much size impact on the output. Unfortunately, there is an impact. I did a quick test with a small program (around 4.7kB compiled code) and it looks like the LoopRotate pass is the main culprit. If `MaxHeaderSize` is set to 0 instead of -1, the code size regression is avoided. The following hacky patch avoids the code size increase: diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp index aa916345954d..99be1926cf34 100644 --- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp +++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp @@ -450,7 +450,7 @@ void PassManagerBuilder::addFunctionSimplificationPasses( // TODO: Investigate promotion cap for O1. MPM.add(createLICMPass(LicmMssaOptCap, LicmMssaNoAccForPromotionCap)); // Rotate Loop - disable header duplication at -Oz - MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1, PrepareForLTO)); + MPM.add(createLoopRotatePass(0/*SizeLevel == 2 ? 0 : -1*/, PrepareForLTO)); // TODO: Investigate promotion cap for O1. MPM.add(createLICMPass(LicmMssaOptCap, LicmMssaNoAccForPromotionCap)); if (EnableSimpleLoopUnswitch) @@ -917,7 +917,7 @@ void PassManagerBuilder::populateModulePassManager( // Re-rotate loops in all our loop nests. These may have fallout out of // rotated form due to GVN or other transformations, and the vectorizer relies // on the rotated form. Disable header duplication at -Oz. - MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1, PrepareForLTO)); + MPM.add(createLoopRotatePass(0 /*SizeLevel == 2 ? 0 : -1*/, PrepareForLTO)); // Distribute loops to allow partial vectorization. I.e. isolate dependences // into separate loop that would otherwise inhibit vectorization. This is So, should all passes just look at the `optsize` and `minsize` attributes instead of the `SizeLevel`? In other words, should `PassManagerBuilder.SizeLevel` be removed and should passes only look at function attributes instead of `SizeLevel`? Because at the moment, it's a weird mix of both. IMHO size level should either all go via function attributes or via a flag, not something in between as it is now. Also, if size level is done via function attributes, why not optimization level? There is already `optnone`. I'm not saying that's better, but right now I don't see the logic in this whole system. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D72404/new/ https://reviews.llvm.org/D72404 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
