http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50339
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-21 12:57:40 UTC --- Created attachment 29517 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29517 gcc48-pr50339.patch Patch that improves 4.8 generated code to one insn better than what 4.7 did, by lowering ASHIFTRT similarly how lower-subreg lowers ASHIFT and LSHIFTRT already. On this testcase the difference is between unpatched trunk and patched trunk: - movq %rdi, %r9 - movq %rsi, %rdi - movq %rsi, %r10 - sarq $63, %rdi - movq %rdi, %rcx - xorq %r9, %rcx - movq %rcx, %rax - movq %r10, %rcx - xorq %rdi, %rcx - subq %rdi, %rax - movq %rcx, %rdx - sbbq %rdi, %rdx + movq %rsi, %rax + sarq $63, %rax + movq %rax, %r9 + xorq %rax, %rdi + xorq %r9, %rsi + movq %rdi, %rax + movq %rsi, %rdx + subq %r9, %rax + sbbq %r9, %rdx i.e. 4 moves instead of former 7 (no idea why RA chooses to do this shift on %rax (i.e. first move %rsi to %rax, then shift %rax, then move %rax to %r9), instead of copying %rsi to %r9 and shifting %r9, that would mean one less move). Even smaller code would probably need different expansion or much smarter register allocation. Anyway, also tested: __int128_t f1 (__int128_t a) { return a >> 67; } __int128_t f2 (__int128_t a) { return a >> 64; } __int128_t f3 (__int128_t a) { return a >> 127; } __uint128_t f4 (__uint128_t a) { return a >> 67; } __uint128_t f5 (__uint128_t a) { return a >> 64; } __uint128_t f6 (__uint128_t a) { return a >> 127; } on x86_64 and the difference at -O2 is: - movq %rsi, %rax movq %rsi, %rdx + movq %rsi, %rax sarq $63, %rdx sarq $3, %rax for f1, - movq %rsi, %rdx movq %rsi, %rax - sarq $63, %rdx + cqto for f2 and + sarq $63, %rsi movq %rsi, %rdx - sarq $63, %rdx - movq %rdx, %rax + movq %rsi, %rax for f3, so either no pessimization, or small improvement. On: long long int f1 (long long int a) { return a >> 35; } long long int f2 (long long int a) { return a >> 32; } long long int f3 (long long int a) { return a >> 63; } unsigned long long int f4 (unsigned long long int a) { return a >> 35; } unsigned long long int f5 (unsigned long long int a) { return a >> 32; } unsigned long long int f6 (unsigned long long int a) { return a >> 63; } for -O2 -m32 the improvements are even better, for f1: - movl 8(%esp), %edx - movl %edx, %eax - movl %eax, %edx - sarl $31, %edx + movl 8(%esp), %eax + cltd sarl $3, %eax and for f2: - movl 8(%esp), %edx - movl %edx, %eax - movl %eax, %edx - sarl $31, %edx + movl 8(%esp), %eax + cltd (no difference for f3).
