[CHERI] Fix a crash during codegen of __builtin_offsetof.

Occurs only when getting an offset of an item of an array member
when the index cannot be constant evaluated and the type is
__intcap. We need to extract the addr part of the cap before it
is used as an index.

Author: veselypeta

clang/lib/CodeGen/CGExprScalar.cpp

@@ -3974,6 +3974,8 @@ Value *ScalarExprEmitter::VisitOffsetOfExpr(OffsetOfExpr *E) {
       // Compute the index
       Expr *IdxExpr = E->getIndexExpr(ON.getArrayExprIndex());
       llvm::Value* Idx = CGF.EmitScalarExpr(IdxExpr);
+      if (Idx->getType()->isPointerTy())
+        Idx = CGF.getCapabilityIntegerValue(Idx);
       bool IdxSigned = IdxExpr->getType()->isSignedIntegerOrEnumerationType();
       Idx = Builder.CreateIntCast(Idx, ResultType, IdxSigned, "conv");
 

clang/test/CodeGen/cheri/cheri-builtin-offsetof.c

@@ -0,0 +1,53 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 2
+// RUN: %riscv32_cheri_purecap_cc1 -o - -emit-llvm -O0 -disable-O0-optnone %s | \
+// RUN:   opt -passes=mem2reg,instcombine -S -o - | FileCheck %s --check-prefixes=RV32
+// RUN: %riscv64_cheri_purecap_cc1 -o - -emit-llvm -O0 -disable-O0-optnone %s | \
+// RUN:   opt -passes=mem2reg,instcombine -S -o - | FileCheck %s --check-prefixes=RV64
+
+struct foo {
+  unsigned int dummy;
+  void *children[2];
+};
+
+// RV32-LABEL: define dso_local i32 @off_with_intcap
+// RV32-SAME: (ptr addrspace(200) noundef [[BIT:%.*]]) addrspace(200) #[[ATTR0:[0-9]+]] {
+// RV32-NEXT:  entry:
+// RV32-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cheri.cap.address.get.i32(ptr addrspace(200) [[BIT]])
+// RV32-NEXT:    [[AND:%.*]] = shl i32 [[TMP0]], 3
+// RV32-NEXT:    [[TMP1:%.*]] = and i32 [[AND]], 8
+// RV32-NEXT:    [[TMP2:%.*]] = add nuw nsw i32 [[TMP1]], 8
+// RV32-NEXT:    ret i32 [[TMP2]]
+//
+// RV64-LABEL: define dso_local i64 @off_with_intcap
+// RV64-SAME: (ptr addrspace(200) noundef [[BIT:%.*]]) addrspace(200) #[[ATTR0:[0-9]+]] {
+// RV64-NEXT:  entry:
+// RV64-NEXT:    [[TMP0:%.*]] = call i64 @llvm.cheri.cap.address.get.i64(ptr addrspace(200) [[BIT]])
+// RV64-NEXT:    [[AND:%.*]] = shl i64 [[TMP0]], 4
+// RV64-NEXT:    [[TMP1:%.*]] = and i64 [[AND]], 16
+// RV64-NEXT:    [[TMP2:%.*]] = add nuw nsw i64 [[TMP1]], 16
+// RV64-NEXT:    ret i64 [[TMP2]]
+//
+unsigned long off_with_intcap(unsigned __intcap bit) {
+  return __builtin_offsetof(struct foo, children[bit & 1]);
+}
+
+// RV32-LABEL: define dso_local i32 @off_with_int
+// RV32-SAME: (i32 noundef [[BIT:%.*]]) addrspace(200) #[[ATTR0]] {
+// RV32-NEXT:  entry:
+// RV32-NEXT:    [[AND:%.*]] = shl i32 [[BIT]], 3
+// RV32-NEXT:    [[TMP0:%.*]] = and i32 [[AND]], 8
+// RV32-NEXT:    [[TMP1:%.*]] = add nuw nsw i32 [[TMP0]], 8
+// RV32-NEXT:    ret i32 [[TMP1]]
+//
+// RV64-LABEL: define dso_local i64 @off_with_int
+// RV64-SAME: (i32 noundef signext [[BIT:%.*]]) addrspace(200) #[[ATTR0]] {
+// RV64-NEXT:  entry:
+// RV64-NEXT:    [[AND:%.*]] = shl i32 [[BIT]], 4
+// RV64-NEXT:    [[TMP0:%.*]] = and i32 [[AND]], 16
+// RV64-NEXT:    [[NARROW:%.*]] = add nuw nsw i32 [[TMP0]], 16
+// RV64-NEXT:    [[TMP1:%.*]] = zext nneg i32 [[NARROW]] to i64
+// RV64-NEXT:    ret i64 [[TMP1]]
+//
+unsigned long off_with_int(unsigned int bit) {
+  return __builtin_offsetof(struct foo, children[bit & 1]);
+}