http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52876
Bug #: 52876 Summary: [x32] - Sign extend 32 to 64bit then clear upper 32bits fails O1 or higher Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target AssignedTo: unassig...@gcc.gnu.org ReportedBy: steffen-schm...@siemens.com Created attachment 27096 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27096 Example code v1 This function sign extends an x32 32bit void* into a 64bit long long global variable. Afterwards it zeros upper 32bits using binary AND. long long li; long long testfunc(void* addr) __attribute__ ((noinline)); long long testfunc(void* addr) { li = (long long)(int)addr; // next line is important, if the address from pv // has been sign extended with leading ONEs li &= 0xffffffff; return li; } int main (void) { volatile long long rv_test; rv_test = testfunc((void*)0x87651234); return 0; } The following examples are called with parameter (void*)0x87651234 Compiled with gcc 4.7.0 -mx32 -O0 produces: movq %rdi, %rax movl %eax, -4(%rbp) movl -4(%rbp), %eax cltq #sign extend EAX to RAX, e.g. 0x87651234 to 0xffffffff87651234 movq %rax, li(%rip) movq li(%rip), %rax andl $4294967295, %eax #the AND 0xffffffff, producing 0x0000000087651234 movq %rax, li(%rip) movq li(%rip), %rax Compiled with gcc 4.7.0 -mx32 -O1 produces: movslq %edi, %rax #this is only a sign extend producing 0xffffffff87651234 #the zero-ing of the upper 32bits is missing. movq %rax, li(%rip) Compiled with gcc 4.6.3 (x32 branch) -mx32 -O1 produces: movq %rdi, %rax #Here the sign extend and the AND are combined to a mov #the results in 0x0000000087651234 which is correct. movq %rdi, li(%rip) It seems that somehow gcc 4.7.0 "forgets" to clear the upper bits, which gcc 4.6.3 branch x32 did correctly. The effect only happens in a stand-alone function and using a global variable, when inlining, the result is correct. I've attached 3 versions of the test, all failing in O1 or higher. The first using a global variable, the second a union and a local variable, the third using a variadic function.