Simple X32 miscompilation

[henke96]

test.c:

void test(void) {
    *(int *)0x80000000 = 0x123;
}

Compile with: clang -target x86_64-unknown-linux-gnux32 -O1 -c test.c

objdump test.o --disassemble=test shows:

00000000 <test>:
   0:   c7 04 25 00 00 00 80    movl   $0x123,0xffffffff80000000
   7:   23 01 00 00 
   b:   c3                      retq 

It stores the value to address 0xffffffff80000000 instead of the expected 0x80000000.

Seems to only happen with optimizations enabled, and only if bit 32 is set in the address.

[llvmbot]

@llvm/issue-subscribers-backend-x86

[RKSimon]

CC @hvdijk

[hvdijk]

Confirmed. I was initially thinking this is unlikely to affect real programs since all symbols are guaranteed to be located in the low 2GB virtual address space, but no such restriction is present for mmap with MAP_FIXED: (int *)0x80000000 can be made a valid pointer that way.

[hvdijk]

https://reviews.llvm.org/D124406