http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52352
Bug #: 52352 Summary: [x32] - Wrong code to access addresses 0x80000000 to 0xFFFFFFFF using registers Classification: Unclassified Product: gcc Version: 4.6.3 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target AssignedTo: unassig...@gcc.gnu.org ReportedBy: steffen-schm...@siemens.com Created attachment 26727 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26727 C code resulting in wrong instructions when compiled with -mx32 and -O2 or higher The example C code defines a structure in memory at address 0xFEE00000 Compilation of this example with -mx32 and -O2 or higher (seems to be related to -fgcse) results in faulty assembler instructions. For accessing several members of the structure loads the signed address of one member of the structure (-18874360 = 0xFEE00008) into register %rax. This actually results in a 64bit negative address of 0xFFFFFFFFFEE00008, which is incorrect when later using complete %rax register for memory access. // -mx32 -O3 movq $-18874360, %rax movl (%rax), %edx xorb %al, %al movl %edx, 4(%eax) When optimizing with -O1 register %eax is used instead of %rax which results in correct behaviour: // -mx32 -O1 movl $-18874368, %eax movl 8(%eax), %edx movl %edx, 4(%eax) The x64 compiler produces correct code with -O1 and -O3 loading the address into %eax not %rax: // -m64 -O1 movl $4276092928, %eax movl 8(%rax), %edx movl %edx, 4(%rax) // -m64 -O3 movl $4276092936, %eax movl (%rax), %edx xorb %al, %al movl %edx, 4(%rax)