[clang][bytecode] Fix unknown size arrays crash in clang bytecode

[osamakader]

Fixes #153948

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Osama Abdelkader (osamakader)

Changes

This fixes issue #153948 where clang crashes with assertion failure 'Array of unknown size' when evaluating strlen() on external const char[] declarations.

The issue was in evaluateStrlen() which called getNumElems() on unknown size arrays, leading to an assertion in Descriptor::getSize().

Fix: Add check for isUnknownSizeArray() before calling getNumElems() to gracefully handle unknown size arrays by returning false (indicating strlen cannot be evaluated at compile time).

Tested with the reproducer from the GitHub issue.

---

Full diff: https://github.com/llvm/llvm-project/pull/160015.diff

1 Files Affected:

• (modified) clang/lib/AST/ByteCode/Context.cpp (+5)

diff --git a/clang/lib/AST/ByteCode/Context.cpp b/clang/lib/AST/ByteCode/Context.cpp
index cfda6e8ded760..f9bc3906beec1 100644
--- a/clang/lib/AST/ByteCode/Context.cpp
+++ b/clang/lib/AST/ByteCode/Context.cpp
@@ -245,6 +245,11 @@ bool Context::evaluateStrlen(State &Parent, const Expr *E, uint64_t &Result) {
     if (!FieldDesc->isPrimitiveArray())
       return false;
 
+    // Handle unknown size arrays - we can't determine the length at compile time
+    if (Ptr.isUnknownSizeArray()) {
+      return false;
+    }
+
     unsigned N = Ptr.getNumElems();
     if (Ptr.elemSize() == 1) {
       Result = strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), N);

[Fznamznon]

Hi, thank you for the fix, could you please add a test? I think a test using __builtin_strlen and a similar array would be appropriate. I think it can be similar to tests that reside in clang/test/AST/ByteCode/builtin-functions.cpp test file.

[tbaederr]

Tested with the reproducer from the GitHub issue.

Add it as a test then?

[osamakader]

The test is added, thanks for the review.

[tbaederr]

This works:

extern const char s[];
void foo(char *x)
{
    char *r;
    __builtin_strcpy(r, s);
}

[tbaederr]

The description of this issue should say "Fixes #153948" at the end so it gets properly closed.

[Fznamznon]

Do you need us to merge that for you?

[osamakader]

@Fznamznon yes please. I'm not able to do it myself :) Thanks.

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