Re: Poor man's JIT compiler

[Dean Anderson <dean at av8 dot com>]

I suspect it does this because of instruction alignment and pipelining
issues.   Why are you trying to turn off alignment?

You might try adding a nop after each one. 

		--Dean

On Tue, 1 Sep 2009, Robert Bernecky wrote:

> I'm trying to get gcc version 4.3.2 to emit X86-64 code
> fragments that I can catenate to perform my own JIT
> compilation, but the compiler is being recalcitrant.
> 
> (I was using a jump table, but its performance was underwhelming.)
> 
> Roughly, what I've done is to create a set of code fragments,
> with labels so that I can determine their address ( via &&label)
> and length. E.g.,
> 
> topLoad1:  reg1 = x[i];
> botLoad1:
> 
> topLoad2:  reg2 = y[i];
> botLoad2:
> 
> topAdd:    regz = reg1 + reg2;
> BotAdd:
> 
> topStore:  z[i] = regz;
> botStore:
> 
> Then, I have a table of fragment addresses (topLoad1, topLoad2, etc.)
> and lengths (botLoad1-topLoad1, botLoad2-topLoad2), and a
> (unknown statically) list of fragments to be assembled to build
> working code, e.g.:
> 
>   (Load2, Load1, Add, Store, Loop)
> 
> I assemble the fragments into a code buffer and jump to it,
> or so the story goes. Unfortunately, what I'm seeing in the
> generated code fragments is not fun:
> 
> 1. GCC sometimes, but NOT always, inserts jumps to the next
>     fragment. E.g.:
> 
> ----------------------------------------------
> 
> .L46:
>          .loc 2 34 0
>          movq    -264(%rbp), %rax
>          movq    %rax, -40(%rbp)
> .L47:
> .L7:
>          .loc 2 40 0
>          movl    %r8d, %eax
>          jmp     .L48
> .L6:
> .L48:
>          .loc 2 43 0
>          movl    %r11d, %ecx
> .L49:
> .L50:
> ----------------------------------------------
> 
> Note the jmp .L48. If GCC always inserted a jump, I could
> remove it, or if it never inserted the jump, I'd be even
> happier, but it only does it now and then. I tried adding
> my own jumps to force this:
> 
> topLoad2:  reg2 = y[i];
>             goto botLoad2;
> botLoad2:
> 
> but GCC removed them. And inserted others.
> 
> Today, I'm using these compiler options:
> 
> gcc  -O0 -ggdb -mtune=opteron -fno-align-labels -fno-align-jumps
> 
> So, I welcome suggestions on how to solve or work around these
> problems. Or even a completely different approach.
> 
> Thanks,
> Robert
> 
> 
> 
> 

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