[jit] Don't emit a redundant move in lazy fetch trampolines on AMD64.

[vkargov]

Hello,

This simple patch is meant to remove meaningless movq %rdi, %rdi instructions generated in rgctx fetch trampolines.
For instance, jitting csc.exe would cause 37 such moves to appear.

A more general approach would be making *_mov_reg_reg skip on move construction if it sees the source and the destination are the same, but I'm cautious that some of the existing code may be somehow relying on the expectation that a dummy instruction will always be built. (it doesn't appear so, though)

[lewurm]

how about you are trying your suggested general approach first? submit a PR and let us see what breaks 🙂

[vargaz]

It would be better to just use an if () instead of an #ifdef, it looks better, and the compiler will optimize it away the same.

[vargaz]

Codegen macros should generate code, they should not do optimization.

[vkargov]

Thanks for the feedback, I've replaced the macro with an if as suggested.

@lewurm
I experimented with that general approach prior to opening this PR and it did not break in an obvious way, i.e. it compiled and all tests passed. However, I'm not completely certain it wouldn't break on some edge case.
Either way, I found out that all redundant moves are generated by that one function this patch is meant to fix, so for all practical purposes it should get rid of the only known cause.
Sorry, I probably should have been more verbose on this from the beginning.

[lewurm]

@monojenkins rebase

[monojenkins]

cannot rebase:

• "Linux i386" state is "failure"
• "Linux x64" state is "failure"
• "OS X i386" state is "failure"
• "OS X x64" state is "success"
• "Windows i386" state is "success"
• "Windows x64" state is "failure"

[monojenkins]

cannot rebase:

• "Linux i386" state is "failure"
• "Linux x64" state is "success"
• "OS X i386" state is "failure"
• "OS X x64" state is "success"
• "Windows i386" state is "success"
• "Windows x64" state is "success"

[monojenkins]

cannot rebase:

• "Linux i386" state is "success"
• "Linux x64" state is "success"
• "OS X i386" state is "failure"
• "OS X x64" state is "success"
• "Windows i386" state is "success"
• "Windows x64" state is "success"