Re: c/8387: [3.3 regression] -finline-limit behavior changed; newinlining flags need doc

[Steven Bosscher <s dot bosscher at student dot tudelft dot nl>]

Op di 11-02-2003, om 21:14 schreef Steven Bosscher:
> This also happens on -O1 -fpic on i586-pc-linux-gnu.  It disappears when
> I comment out "flag_crossjumping = 1;" in toplev.c, so this bug probably
> is somewhere in cleanup_cfg()
> 
> Greetz
> Steven
> 

Yup, crossjumping or cfgcleanup after crossjumping seems to be the cause
of this.

For this code for example, the problem is that a whole jump table for
one of the switch statements is wiped out 

---- 8< ----
int *g;

int main (void)
{
  switch (*g)
    {
    case 0:
      {
        switch (*g)
          {
          case 0: *g = 1; break;
          case 1:
          case 2: *g = 1; break;
          case 3:
          case 4: *g = 1; break;
          }
        break;
      }
    case 1:
      {
        switch (*g)
          {
          case 0: *g = 1; break;
          case 1:
          case 2: *g = 1; break;
          case 3:
          case 4: *g = 1; break;
          }
      }
    }
  return 0;
}
---- 8< ----
steven# gcc-3.4 -v   
Reading specs from /opt/experimental/lib/gcc-lib/i586-pc-linux-gnu/3.4/specs
Configured with: ../gcc-trunk/configure --disable-nls --with-gnu-as --with-gnu-ld --prefix=/opt/experimental --program-suffix=-3.4 --enable-languages=c
Thread model: posix
gcc version 3.4 20030211 (experimental)
steven# gcc-3.4 c7887_PR.c -O -fpic -save-temps
c7887_PR.o: In function `main':
c7887_PR.o(.text+0x3b): undefined reference to `.L10'
collect2: ld returned 1 exit status

Look at a piece of the assembly diff:


.L3:					.L3:
movl	g@GOT(%ecx), %eax		movl	g@GOT(%ecx), %eax
movl	(%eax), %eax			movl	(%eax), %eax
cmpl	$4, (%eax)			cmpl	$4, (%eax)
ja	.L2				ja	.L2
movl	(%eax), %eax			movl	(%eax), %eax
leal	.L10@GOTOFF(%ecx), %edx		leal	.L10@GOTOFF(%ecx), %edx
jmp	.L19			     |	movl	(%edx,%eax,4), %eax
				     >	addl	%ecx, %eax
				     >	jmp	*%eax
				     >	.section	.rodata
				     >	.align 4
				     >	.align 4
				     >	.L10:
				     >	.long	.L5@GOTOFF
				     >	.long	.L7@GOTOFF
				     >	.long	.L7@GOTOFF
				     >	.long	.L9@GOTOFF
				     >	.long	.L9@GOTOFF
				     >	.text
				     >	.L5:
				     >	movl	g@GOT(%ecx), %eax
				     >	movl	(%eax), %eax
				     >	movl	$1, (%eax)
				     >	jmp	.L2
				     >	.L7:
				     >	movl	g@GOT(%ecx), %eax
				     >	movl	(%eax), %eax
				     >	movl	$1, (%eax)
				     >	jmp	.L2
				     >	.L9:
				     >	movl	g@GOT(%ecx), %eax
				     >	movl	(%eax), %eax
				     >	movl	$1, (%eax)
				     >	jmp	.L2
.L11:					.L11:
movl	g@GOT(%ecx), %eax		movl	g@GOT(%ecx), %eax
movl	(%eax), %eax			movl	(%eax), %eax
cmpl	$4, (%eax)			cmpl	$4, (%eax)
ja	.L2				ja	.L2
movl	(%eax), %eax			movl	(%eax), %eax
leal	.L18@GOTOFF(%ecx), %edx		leal	.L18@GOTOFF(%ecx), %edx
.L19:				     <
movl	(%edx,%eax,4), %eax		movl	(%edx,%eax,4), %eax

The label that's missing refers to the jump table, but it's gone...

This is for the first select block with five cases.  The second block
results in a jumptable to 5 identical labels with crossjumping enabled.

To me this is black magic :-)  Hope it helps somebody to fix this bug.

Greetz
Steven