useless trampoline address alignment

useless trampoline address alignment

[Philippe De Muyter]

Testing trampolines, I have found some strange code generated to get the
address of the nested function.  The code aligns an address that should
be generated correctly aligned.  This is on m68k-motorola-sysv, but I surmise
it is target-independent. (this is with egcs-971127, but I am waiting
for the .cexp/14 chars filename problem fix before downloading a newer version)

Philippe De Muyter

Here is the testcase :

f()
	{
	int	h(){}
	char	g[4096];
	int	i(){}

	a(h);
	a(i);
	}

and the assembler code (m68k); look at <<<<<<<<<'s :

	file	"trampo.c"
gcc2_compiled%:
text
	even
h_%2:
	link.w %fp,&-4
	mov.l %a0,-4(%fp)
	unlk %fp
	rts
	even
i_%5:
	link.w %fp,&-4
	mov.l %a0,-4(%fp)
	unlk %fp
	rts
	even
global f
f:
	link.w %fp,&-4124
	mov.l %a3,-(%sp)
	mov.l %a2,-(%sp)
	lea -4123(%fp),%a3	<<<<<<<<< ???
	lea -4122(%fp),%a0	<<<<<<<<< That's the trampoline's address
	mov.w &8316,(%a0)
	mov.l %fp,2(%a0)
	mov.w &20217,6(%a0)
	mov.l &i_%5,8(%a0)
	lea -4108(%fp),%a0
	mov.w &8316,(%a0)
	mov.l %fp,2(%a0)
	mov.w &20217,6(%a0)
	mov.l &h_%2,8(%a0)
	pea -4108(%fp)
	lea a,%a2
	jsr (%a2)
	mov.l %a3,%d0		<<<<<<<<<< ???
	addq.l &1,%d0		<<<<<<<<<< ???
	mov.l &-2,%d1		<<<<<<<<<< ???
	and.l %d0,%d1		<<<<<<<<<< ???
	mov.l %d1,-(%sp)	<<<<<<<<<< Should be : pea -4122(%fp)
	jsr (%a2)
	mov.l -4132(%fp),%a2
	mov.l -4128(%fp),%a3
	unlk %fp
	rts

Re: useless trampoline address alignment

[Jeffrey A Law]

  In message < 199712191014.LAA10079@mail.macqel.be >you write:
  > Testing trampolines, I have found some strange code generated to get the
  > address of the nested function.  The code aligns an address that should
  > be generated correctly aligned.  This is on m68k-motorola-sysv, but I surmi se
  > it is target-independent.
I think the code is just trying to be overly generic, and as a result
isn't as efficient as it could be on some machines like the m68k.

I believe the alignment is necessary for targets that do not define
a template (do we have any such targets?)

I don't think this is a very serious problem, so I'm not going to
dive further into it.  However, if you want to try and optimize away
the useless rounding instructions I wouldn't object.

jeff