http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47582
Summary: Combine chains of movl into movq Product: gcc Version: 4.6.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: rtl-optimization AssignedTo: unassig...@gcc.gnu.org ReportedBy: tony.popple...@gmail.com The following C code (adapted from http://stackoverflow.com/questions/4544804/in-what-cases-should-i-use-memcpy-over-standard-operators-in-c) shows that adjacent sequences of movl could be combined into movq. extern float a[5]; extern float b[5]; int main() { #if defined(M1) a[0] = b[0]; a[1] = b[1]; a[2] = b[2]; a[3] = b[3]; a[4] = b[4]; #elif defined(M2) memcpy(a, b, 5*sizeof(float)); #endif } When compiled with "-O2 -fomit-frame-pointer" on GCC 4.3.5, 4.4.5, 4.5.2 and 4.6.0 (20110129), the following asm is produced for the -DM1 branch: movl b(%rip), %eax movl %eax, a(%rip) movl b+4(%rip), %eax movl %eax, a+4(%rip) movl b+8(%rip), %eax movl %eax, a+8(%rip) movl b+12(%rip), %eax movl %eax, a+12(%rip) movl b+16(%rip), %eax movl %eax, a+16(%rip) ret However for the -DM2 branch, the memcpy implementation shows that this can be done more efficiently: movq b(%rip), %rax movq %rax, a(%rip) movq b+8(%rip), %rax movq %rax, a+8(%rip) movl b+16(%rip), %eax movl %eax, a+16(%rip) ret I presume that the memcpy is being done in hand-written asm? If so, then once this enhancment is done, then presumably that portion of memcpy code could be converted to C code and be just as efficient.