https://bugs.llvm.org/show_bug.cgi?id=44718
Bug ID: 44718
Summary: Missed optimization: aliasing problem in a struct when
a member array is indexed with 64-bit int
Product: clang
Version: unspecified
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: michel.bern...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org,
neeil...@live.com, richard-l...@metafoo.co.uk
Tested with clang 9.0.0: x86_64-pc-windows-msvc
Compiled with '-O2'.
-----C CODE-----
#include <stdint.h>
struct S {
uint32_t a;
uint32_t array[32];
};
uint32_t test1(struct S* ptr, uint32_t index)
{
ptr->a = 5;
ptr->array[index] = 4;
return ptr->a;
}
uint32_t test2(struct S* ptr, uint64_t index)
{
ptr->a = 5;
ptr->array[index] = 4;
return ptr->a;
}
----------------
Produced assembly:
test1:
mov dword ptr [rcx], 5
mov eax, edx
mov dword ptr [rcx + 4*rax + 4], 4
mov eax, 5
ret
test2:
mov dword ptr [rcx], 5
mov dword ptr [rcx + 4*rdx + 4], 4
mov eax, dword ptr [rcx]
ret
In 'test2', the first store is not forwarded, as if the array store could
overwrite the first member. This doesn't happen in 'test1' where the index is
32-bits.
If I'm not mistaken, any index outside the array range is undefined behaviour,
regardless of the width of the index, so I don't see any reason for a member
store to alias another member.
I also tested with GCC and in both cases the first store is correctly
forwarded.
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