https://bugs.llvm.org/show_bug.cgi?id=42668
Bug ID: 42668
Summary: Clang generates overaligned load of thrown object
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Keywords: ABI, miscompilation
Severity: normal
Priority: P
Component: C++
Assignee: diogo.samp...@arm.com
Reporter: diogo.samp...@arm.com
CC: blitzrak...@gmail.com, dgre...@apple.com,
erik.pilking...@gmail.com, llvm-bugs@lists.llvm.org,
richard-l...@metafoo.co.uk
For the C++ code:
===
#include <arm_neon.h>
int main(void) {
try {
throw vld1q_u64(((const uint64_t[2]){1, 2}));
} catch (uint64x2_t exc) {
return 0;
}
return 1;
}
====
and command:
clang --target=arm-arm-none-eabi -march=armv8-a -c test_upstream.cpp -o - -Os
-o - -S -emit-llvm
====
we obtain this:
entry:
%exception = tail call i8* @__cxa_allocate_exception(i32 16) #2
%0 = bitcast i8* %exception to <2 x i64>*
store <2 x i64> <i64 1, i64 2>, <2 x i64>* %0, align 16, !tbaa !5
========
Which, passing to llc will generate:
vld1.64 {d16, d17}, [r1]
vst1.64 {d16, d17}, [r0:128] << Not 128, but 64!!!
========
The store assumes an alignment of 16. However, %exception has alignment 64 as
defined in
https://github.com/llvm-mirror/libunwind/blob/master/include/unwind.h
where we have:
struct _Unwind_Exception {
...
} __attribute__((__aligned__)); //<< For the ARM ABI the largest alignment is 8
(64 bits).
I do believe that the missing "align 8" in %exception should be the root of the
error, as that should be passed further to %0 and, finally, to the store.
I also believe that clang should not assume data-width alignment when the
pointer is a bitcast from a smaller data-type.
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