[clang] [clang][codegen] Add a verifier IR pass before any further passes. (PR #68015)

[Matheus Izvekov via cfe-commits]

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@@ -1020,21 +1022,23 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
     }
 
     if (CodeGenOpts.FatLTO) {
-      MPM = PB.buildFatLTODefaultPipeline(Level, PrepareForThinLTO,
-                                          PrepareForThinLTO ||
-                                              shouldEmitRegularLTOSummary());
+      MPM.addPass(PB.buildFatLTODefaultPipeline(
+          Level, PrepareForThinLTO,
+          PrepareForThinLTO || shouldEmitRegularLTOSummary()));
     } else if (PrepareForThinLTO) {
-      MPM = PB.buildThinLTOPreLinkDefaultPipeline(Level);
+      MPM.addPass(PB.buildThinLTOPreLinkDefaultPipeline(Level));
     } else if (PrepareForLTO) {
-      MPM = PB.buildLTOPreLinkDefaultPipeline(Level);
+      MPM.addPass(PB.buildLTOPreLinkDefaultPipeline(Level));
     } else {
-      MPM = PB.buildPerModuleDefaultPipeline(Level);
+      MPM.addPass(PB.buildPerModuleDefaultPipeline(Level));
     }
   }
 
   // Add a verifier pass if requested. We don't have to do this if the action
   // requires code generation because there will already be a verifier pass in
   // the code-generation pipeline.
+  // Since we already added a verifier pass above, this
+  // might even not run the analysis, if previous passes caused no changes.
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mizvekov wrote:

Yeah, that's what it says and is implemented here: https://github.com/llvm/llvm-project/blob/0b07b06effe5fdf779b75bb5ac6cf15e477cb0be/llvm/include/llvm/IR/PassManager.h#L772

And it matches what I see when running the pass-manager debugger on O0 vs O1 builds, as you see on the changes affecting the `clang/test/CodeGen/lto-newpm-pipeline.c` test. You can see for O0 that the `VerifierPass` is executed a second time near the bottom, but that doesn't trigger a `VerifierAnalysis` run.

https://github.com/llvm/llvm-project/pull/68015