http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50440
Bug #: 50440 Summary: 128 bit unsigned int subtraction generates too many register moves Classification: Unclassified Product: gcc Version: 4.6.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization AssignedTo: unassig...@gcc.gnu.org ReportedBy: schnet...@gmail.com I want to perform 128 bit integer arithmetic, and I am declaring my type like this: {{{ typedef unsigned int uint128_t __attribute__((mode(TI))); uint128_t add (uint128_t x, uint128_t y) { return x+y; } uint128_t sub (uint128_t x, uint128_t y) { return x-y; } }}} This is on an Intel Xeon processor in x86_64 mode. I build with the command gcc-4.6.1 -O3 -march=native -S sub128.c and I find that, while the "add" routine looks optimal, the "sub" routine has several unnecessary register moves: {{{ add: movq %rdx, %rax movq %rcx, %rdx addq %rdi, %rax adcq %rsi, %rdx ret sub: movq %rsi, %r10 movq %rdi, %rsi subq %rdx, %rsi movq %r10, %rdi sbbq %rcx, %rdi movq %rsi, %rax movq %rdi, %rdx ret }}}