Invalid LLVM for constant array reference

[yugr]

Compiling the following code with trunk CIR

int a[16];
int *const p = a;

void foo() {
  p[0];
}

generates invalid LLVM IR:

loc("tmp.c":5:3): error: 'llvm.mlir.constant' op only supports integer, float, string or elements attributes

Removal of const fixes it.

[yugr]

The issue is caused by

%0 = llvm.mlir.constant(#cir.global_view<@a> : !cir.ptr<!cir.int<s, 32>>) : !llvm.ptr

which should have been llvm.mlir.addressof instead. This is caused by generation of

%0 = cir.const(#cir.global_view<@a> : !cir.ptr<!s32i>)

rather than cir.get_global in ScalarExprEmitter::VisitDeclRefExpr:

    if (CIRGenFunction::ConstantEmission Constant = CGF.tryEmitAsConstant(E)) {
      return CGF.buildScalarConstant(Constant, E);
    }

[yugr]

A simple workaround is to handle this at lowering stage:

diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
index 076c0eb814b4..8b23213c70f2 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
@@ -1100,6 +1100,13 @@ public:
       return op.emitError() << "unsupported constant type " << op.getType();
 
+    if (auto GV = dyn_cast<mlir::cir::GlobalViewAttr>(attr)) {
+      auto newOp = lowerCirAttrAsValue(op, GV, rewriter, getTypeConverter());
+      rewriter.replaceOp(op, newOp);
+      return mlir::success();
+    }
+
     rewriter.replaceOpWithNewOp<mlir::LLVM::ConstantOp>(
         op, getTypeConverter()->convertType(op.getType()), attr);

but I suspect we may not want to generate the problematic cir.const at all.

[yugr]

Fixed by #363