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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Hi all,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>x86 processors use macro-op fusion to merge together two instructions and execute them as one. So it's beneficial for the compiler to emit them as a pair.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Currently only compare and jump instructions get fused though. And I was wondering whether it also makes sense to fuse move and arithmetic instructions together, to form non-destructive instructions (which x86 lacks for regular instructions). For instance:<o:p></o:p></p><p class=MsoNormal style='line-height:12.0pt'><span style='font-size:8.5pt;font-family:"Arial","sans-serif";color:black'> 8B C3 mov eax, ebx <o:p></o:p></span></p><p class=MsoNormal style='line-height:12.0pt'><span style='font-size:8.5pt;font-family:"Arial","sans-serif";color:black'> 03 C1 add eax, ecx<o:p></o:p></span></p><p class=MsoNormal>becomes<o:p></o:p></p><p class=MsoNormal> <span class=apple-style-span><span style='font-size:8.5pt;font-family:"Arial","sans-serif";color:black'>8B C3 03 C1 add eax, ebx, ecx</span></span><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>There's no difference in the binary encoding; it's just considered one instruction at a logical level and inside the hardware (I'm assuming x86's RISC internals actually use non-destructive micro-operations).<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>So my question is, how do I define these fused instructions in LLVM? And how would I be able to estimate the potential speedup?<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thanks,<o:p></o:p></p><p class=MsoNormal>Nicolas<o:p></o:p></p></div></body></html>