[Matrix] Preserve signedness when extending matrix index expression. (#103044)

As per [1] the indices for a matrix element access operator shall have
integral or unscoped enumeration types and be non-negative. At the
moment, the index expression is converted to SizeType irrespective of
the signedness of the index expression. This causes implicit sign
conversion warnings if any of the indices is signed.

As per the spec, using signed types as indices is allowed and should not
cause any warnings. If the index expression is signed, extend to
SignedSizeType to avoid the warning.

[1]
https://clang.llvm.org/docs/MatrixTypes.html#matrix-type-element-access-operator

PR: #103044

Author: fhahn

clang/lib/CodeGen/CGExpr.cpp

@@ -4392,13 +4392,24 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
   return LV;
 }
 
+llvm::Value *CodeGenFunction::EmitMatrixIndexExpr(const Expr *E) {
+  llvm::Value *Idx = EmitScalarExpr(E);
+  if (Idx->getType() == IntPtrTy)
+    return Idx;
+  bool IsSigned = E->getType()->isSignedIntegerOrEnumerationType();
+  return Builder.CreateIntCast(Idx, IntPtrTy, IsSigned);
+}
+
 LValue CodeGenFunction::EmitMatrixSubscriptExpr(const MatrixSubscriptExpr *E) {
   assert(
       !E->isIncomplete() &&
       "incomplete matrix subscript expressions should be rejected during Sema");
   LValue Base = EmitLValue(E->getBase());
-  llvm::Value *RowIdx = EmitScalarExpr(E->getRowIdx());
-  llvm::Value *ColIdx = EmitScalarExpr(E->getColumnIdx());
+
+  // Extend or truncate the index type to 32 or 64-bits if needed.
+  llvm::Value *RowIdx = EmitMatrixIndexExpr(E->getRowIdx());
+  llvm::Value *ColIdx = EmitMatrixIndexExpr(E->getColumnIdx());
+
   llvm::Value *NumRows = Builder.getIntN(
       RowIdx->getType()->getScalarSizeInBits(),
       E->getBase()->getType()->castAs<ConstantMatrixType>()->getNumRows());

clang/lib/CodeGen/CGExprScalar.cpp

@@ -2007,8 +2007,8 @@ Value *ScalarExprEmitter::VisitMatrixSubscriptExpr(MatrixSubscriptExpr *E) {
 
   // Handle the vector case.  The base must be a vector, the index must be an
   // integer value.
-  Value *RowIdx = Visit(E->getRowIdx());
-  Value *ColumnIdx = Visit(E->getColumnIdx());
+  Value *RowIdx = CGF.EmitMatrixIndexExpr(E->getRowIdx());
+  Value *ColumnIdx = CGF.EmitMatrixIndexExpr(E->getColumnIdx());
 
   const auto *MatrixTy = E->getBase()->getType()->castAs<ConstantMatrixType>();
   unsigned NumRows = MatrixTy->getNumRows();

clang/lib/CodeGen/CodeGenFunction.h

@@ -4280,6 +4280,7 @@ class CodeGenFunction : public CodeGenTypeCache {
   LValue EmitUnaryOpLValue(const UnaryOperator *E);
   LValue EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
                                 bool Accessed = false);
+  llvm::Value *EmitMatrixIndexExpr(const Expr *E);
   LValue EmitMatrixSubscriptExpr(const MatrixSubscriptExpr *E);
   LValue EmitArraySectionExpr(const ArraySectionExpr *E,
                               bool IsLowerBound = true);

clang/lib/Sema/SemaExpr.cpp

@@ -5033,8 +5033,7 @@ ExprResult Sema::CreateBuiltinMatrixSubscriptExpr(Expr *Base, Expr *RowIdx,
       }
     }
 
-    ExprResult ConvExpr =
-        tryConvertExprToType(IndexExpr, Context.getSizeType());
+    ExprResult ConvExpr = IndexExpr;
     assert(!ConvExpr.isInvalid() &&
            "should be able to convert any integer type to size type");
     return ConvExpr.get();

clang/test/SemaCXX/matrix-index-operator-sign-conversion.cpp

@@ -2,19 +2,16 @@
 
 template <typename T, int R, int C> using m __attribute__((__matrix_type__(R,C))) = T;
 
-// FIXME: should not warn here.
 double index1(m<double,3,1> X, int      i) { return X[i][0]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'int' to 'unsigned long'}}
 
 double index2(m<double,3,1> X, unsigned i) { return X[i][0]; }
 
 double index3(m<double,3,1> X, char     i) { return X[i][0]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'char' to 'unsigned long'}}
 
 double index4(m<double,3,1> X, int      i) { return X[0][i]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'int' to 'unsigned long'}}
 
 double index5(m<double,3,1> X, unsigned i) { return X[0][i]; }
 
 double index6(m<double,3,1> X, char     i) { return X[0][i]; }
-// expected-warning@-1 {{implicit conversion changes signedness: 'char' to 'unsigned long'}}
+
+// expected-no-diagnostics