[LLVMdev] Macro-op fusion experiment

[Nicolas Capens]

Hi all,

 

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.

 

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:

                8B C3 mov eax, ebx 

                03 C1 add eax, ecx

becomes

                8B C3 03 C1 add eax, ebx, ecx

 

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).

 

So my question is, how do I define these fused instructions in LLVM? And how
would I be able to estimate the potential speedup?

 

Thanks,

Nicolas

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