[llvm] Reland "[LTO] Run Argument Promotion before IPSCCP" (PR #111853)

[Nikita Popov via llvm-commits]

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@@ -1828,6 +1828,15 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
     MPM.addPass(PGOIndirectCallPromotion(
         true /* InLTO */, PGOOpt && PGOOpt->Action == PGOOptions::SampleUse));
 
+    // Promoting by-reference arguments to by-value exposes more constants to
+    // IPSCCP.
+    MPM.addPass(
+        createModuleToPostOrderCGSCCPassAdaptor(PostOrderFunctionAttrsPass()));
+    MPM.addPass(
+        createModuleToPostOrderCGSCCPassAdaptor(ArgumentPromotionPass()));
+    MPM.addPass(
+        createModuleToFunctionPassAdaptor(SROAPass(SROAOptions::ModifyCFG)));
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nikic wrote:

You should be able to add SROA to it as well, with a CGSCC to Function pass adaptor. (I think this may also give better results? Otherwise we'll only run SROA after *all* arg promotion, rather than interleaved.)

https://github.com/llvm/llvm-project/pull/111853