https://llvm.org/bugs/show_bug.cgi?id=30229
Bug ID: 30229
Summary: Clang generates stacksave/stackrestore incorrectly
when a loop has VLA and alloca
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: bluechristl...@163.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Consider the following case:
#include <alloca.h>
#include <string.h>
#include <new>
int main(void) {
char *cp = 0;
for (int i = 1; i <= 100; ++i) {
char vla[i];
memset(vla, 0, static_cast<size_t>(i));
if (!cp) cp = new (alloca(1)) char('Q');
}
return *cp == 'Q' ? 55 : 66;
}
Expected return code 55.
Clang emit LLVM IR:
we sill generate @llvm.stackstore and @llvm.stacksave. However, from GCC 6, the
VLA scope is changed when the block has alloca function:
see: https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html
Space allocated with alloca exists until the containing function returns. The
space for a variable-length array is deallocated as soon as the array name's
scope ends, unless you also use alloca in this scope.
Clang should consider this condition. GCC 6 passes this case but Clang failed.
Similar bug: https://llvm.org/bugs/show_bug.cgi?format=multiple&id=16099
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