[clang] [CIR] Lower array globals with many trailing zeros as packed structs (PR #232835)

[Konstantinos Parasyris via cfe-commits]

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@@ -929,6 +932,17 @@ mlir::Value CIRAttrToValue::visitCirAttr(cir::ConstArrayAttr attr) {
 
   // Iteratively lower each constant element of the array.
   if (auto arrayAttr = mlir::dyn_cast<mlir::ArrayAttr>(attr.getElts())) {
+    // Fewer struct members than elements: adjustGlobalTypeForInit grouped the
+    // elements of an array ending in many zeros into one leading array.
+    auto structTy = mlir::dyn_cast<mlir::LLVM::LLVMStructType>(llvmTy);
+    if (structTy && structTy.getBody().size() < arrayAttr.size()) {
+      auto arrayTy = mlir::cast<cir::ArrayType>(attr.getType());
+      auto prefix = cir::ConstArrayAttr::get(
+          cir::ArrayType::get(arrayTy.getElementType(), arrayAttr.size()),
+          arrayAttr);
+      return mlir::LLVM::InsertValueOp::create(rewriter, loc, result,
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koparasy wrote:

Ran it: replayed all 6,672 TUs of a self-build. I have a python script that goes over compile_commands.json with -fclangir, with and without this PR, OG runs as noise floor. 

Total CIR CPU 48,672s → 48,646s (0.999×); per-TU on/off distribution matches OG noise. 

I added a fixme in the last commit. I will merge this in if no objections sometime next week. 

https://github.com/llvm/llvm-project/pull/232835