[clang] [C2y] Fix _Countof handling of VLAs (PR #141621)
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Tue May 27 08:33:08 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang-codegen
@llvm/pr-subscribers-clang
Author: Aaron Ballman (AaronBallman)
<details>
<summary>Changes</summary>
It turns out that getVLASize() does not get you the size of a single dimension of the VLA, it gets you the full count of all elements. This caused _Countof to return invalid values on VLA ranks. Now switched to using getVLAElements1D() instead, which only gets a single dimension.
Fixes #<!-- -->141409
---
Full diff: https://github.com/llvm/llvm-project/pull/141621.diff
2 Files Affected:
- (modified) clang/lib/CodeGen/CGExprScalar.cpp (+11-5)
- (modified) clang/test/C/C2y/n3369_3.c (+54)
``````````diff
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index b9eb12efd6670..0b0428d1e916e 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -3574,20 +3574,26 @@ ScalarExprEmitter::VisitUnaryExprOrTypeTraitExpr(
CGF.EmitIgnoredExpr(E->getArgumentExpr());
}
- auto VlaSize = CGF.getVLASize(VAT);
- llvm::Value *size = VlaSize.NumElts;
-
// For sizeof and __datasizeof, we need to scale the number of elements
// by the size of the array element type. For _Countof, we just want to
// return the size directly.
+ llvm::Value *Size;
if (Kind != UETT_CountOf) {
+ auto VlaSize = CGF.getVLASize(VAT);
+ Size = VlaSize.NumElts;
+
// Scale the number of non-VLA elements by the non-VLA element size.
CharUnits eltSize = CGF.getContext().getTypeSizeInChars(VlaSize.Type);
if (!eltSize.isOne())
- size = CGF.Builder.CreateNUWMul(CGF.CGM.getSize(eltSize), size);
+ Size = CGF.Builder.CreateNUWMul(CGF.CGM.getSize(eltSize), Size);
+ } else {
+ // For _Countof, we don't want the size of the entire VLA, just the
+ // single dimension we're currently looking at.
+ auto VlaSize = CGF.getVLAElements1D(VAT);
+ Size = VlaSize.NumElts;
}
- return size;
+ return Size;
}
}
} else if (E->getKind() == UETT_OpenMPRequiredSimdAlign) {
diff --git a/clang/test/C/C2y/n3369_3.c b/clang/test/C/C2y/n3369_3.c
index d3909b2e5bc92..792c076413372 100644
--- a/clang/test/C/C2y/n3369_3.c
+++ b/clang/test/C/C2y/n3369_3.c
@@ -164,3 +164,57 @@ void test4(int n) {
int y = _Countof(int[n++][7]);
}
+// CHECK-64-LABEL: define dso_local void @test5(
+// CHECK-64-SAME: ) #[[ATTR0]] {
+// CHECK-64-NEXT: [[ENTRY:.*:]]
+// CHECK-64-NEXT: [[J:%.*]] = alloca i32, align
+// CHECK-64-NEXT: [[SAVED_STACK:%.*]] = alloca ptr, align
+// CHECK-64-NEXT: [[A:%.*]] = alloca i32, align
+// CHECK-64-NEXT: [[B:%.*]] = alloca i32, align
+// CHECK-64-NEXT: [[C:%.*]] = alloca i32, align
+// CHECK-64-NEXT: [[D:%.*]] = alloca i32, align
+// CHECK-64-NEXT: store i32 2, ptr [[J]], align
+// CHECK-64-NEXT: [[TMP0:%.*]] = call ptr @llvm.stacksave.p0()
+// CHECK-64-NEXT: store ptr [[TMP0]], ptr [[SAVED_STACK]], align
+// CHECK-64-NEXT: [[VLA:%.*]] = alloca i32, i64 120, align
+// CHECK-64-NEXT: store i32 1, ptr [[A]], align
+// CHECK-64-NEXT: store i32 4, ptr [[B]], align
+// CHECK-64-NEXT: store i32 5, ptr [[C]], align
+// CHECK-64-NEXT: store i32 6, ptr [[D]], align
+// CHECK-64-NEXT: [[TMP1:%.*]] = load ptr, ptr [[SAVED_STACK]], align
+// CHECK-64-NEXT: call void @llvm.stackrestore.p0(ptr [[TMP1]])
+// CHECK-64-NEXT: ret void
+//
+// CHECK-32-LABEL: define dso_local void @test5(
+// CHECK-32-SAME: ) #[[ATTR0]] {
+// CHECK-32-NEXT: [[ENTRY:.*:]]
+// CHECK-32-NEXT: [[J:%.*]] = alloca i32, align
+// CHECK-32-NEXT: [[SAVED_STACK:%.*]] = alloca ptr, align
+// CHECK-32-NEXT: [[A:%.*]] = alloca i32, align
+// CHECK-32-NEXT: [[B:%.*]] = alloca i32, align
+// CHECK-32-NEXT: [[C:%.*]] = alloca i32, align
+// CHECK-32-NEXT: [[D:%.*]] = alloca i32, align
+// CHECK-32-NEXT: store i32 2, ptr [[J]], align
+// CHECK-32-NEXT: [[TMP0:%.*]] = call ptr @llvm.stacksave.p0()
+// CHECK-32-NEXT: store ptr [[TMP0]], ptr [[SAVED_STACK]], align
+// CHECK-32-NEXT: [[VLA:%.*]] = alloca i32, i32 120, align
+// CHECK-32-NEXT: store i32 1, ptr [[A]], align
+// CHECK-32-NEXT: store i32 4, ptr [[B]], align
+// CHECK-32-NEXT: store i32 5, ptr [[C]], align
+// CHECK-32-NEXT: store i32 6, ptr [[D]], align
+// CHECK-32-NEXT: [[TMP1:%.*]] = load ptr, ptr [[SAVED_STACK]], align
+// CHECK-32-NEXT: call void @llvm.stackrestore.p0(ptr [[TMP1]])
+// CHECK-32-NEXT: ret void
+//
+void test5() {
+ // Ensure we're getting the variable-length dimensions correctly. We were
+ // previously returning the size of all VLA dimensions multiplied together
+ // to get the total element count rather than the element count for a single
+ // array rank. See GH141409
+ const int j = 2;
+ int arr[1][j + 2][j + 3][j + 4];
+ int a = _Countof arr;
+ int b = _Countof *arr;
+ int c = _Countof **arr;
+ int d = _Countof ***arr;
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/141621
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