[PATCH] Fix transform_to_exit_first_loop_alt with -g

[Tom de Vries]

Hi,

This patch fixes a problem with -g compilation in 
transform_to_exit_first_loop_alt.

Consider test-case test.c:
...
void
f (int *a, int n)
{
  int i;
  for (i = 0; i < n; ++i)
    a[i] = 1;
}
...

If we add a "checking_verify_ssa (true, true)" call at the end of 
transform_to_exit_first_loop_alt, and we compile with "-g -O2 
-ftree-parallelize-loops=4", we run into this ICE:
...
test.c: In function ‘f’:
test.c:2:1: error: definition in block 5 does not dominate use in block 13
for SSA_NAME: i_10 in statement:
# DEBUG i => i_10
test.c:2:1: internal compiler error: verify_ssa failed
...

Before transform_to_exit_first_loop_alt, the loop looks like:
...
  <bb 11>:

  <bb 5>:
  # ivtmp_22 = PHI <0(11), ivtmp_23(7)>
  i_13 = ivtmp_22;
  # DEBUG i => i_13
  _5 = (long unsigned int) i_13;
  _6 = _5 * 4;
  _8 = a_7(D) + _6;
  *_8 = 1;
  i_10 = i_13 + 1;
  # DEBUG i => i_10
  # DEBUG i => i_10
  if (ivtmp_22 < _1)
    goto <bb 7>;
  else
    goto <bb 6>;

  <bb 7>:
  ivtmp_23 = ivtmp_22 + 1;
  goto <bb 5>;
...


And after transform_to_exit_first_loop_alt, it looks like:
...
  <bb 11>:
  goto <bb 13>;

  <bb 5>:
  # ivtmp_22 = PHI <ivtmp_25(13)>
  i_13 = ivtmp_22;
  # DEBUG i => i_13
  _5 = (long unsigned int) i_13;
  _6 = _5 * 4;
  _8 = a_7(D) + _6;
  *_8 = 1;
  i_10 = i_13 + 1;
  goto <bb 7>;

  <bb 13>:
  # ivtmp_25 = PHI <ivtmp_23(7), 0(11)>
  # DEBUG i => i_10
  # DEBUG i => i_10
  if (ivtmp_25 < _2)
    goto <bb 5>;
  else
    goto <bb 14>;

  <bb 7>:
  ivtmp_23 = ivtmp_22 + 1;
  goto <bb 13>;
...

The ICE triggers because the use of i_10 in debug insn 'DEBUG i => i_10' 
in bb 13 is no longer dominated by the defition of i_10 in bb 5.

The patch fixes the ICE by ensuring that 
gimple_split_block_before_cond_jump really splits before cond_jump, 
instead of after the last nondebug insn before cond_jump, as it does 
now. This behaviour also better matches the rtl implementation of the 
cfghook. Btw, note that the only user of cfghook 
split_block_before_cond_jump is transform_to_exit_first_loop_alt.

[ A similar fix for an openacc variant of this ICE was committed on the 
gomp-4_0-branch: https://gcc.gnu.org/ml/gcc-patches/2015-07/msg00060.html ]

Bootstrapped and reg-tested on x86_64.

OK for trunk?

Thanks,
- Tom
Fix transform_to_exit_first_loop_alt with -g

2015-11-06  Tom de Vries  <t...@codesourcery.com>

	* tree-cfg.c (gimple_split_block_before_cond_jump): Split before
	cond_jump, instead of split after last nondebug insn before cond_jump.
	* tree-parloops.c (transform_to_exit_first_loop_alt): Verify ssa before
	returning.
---
 gcc/tree-cfg.c      | 2 +-
 gcc/tree-parloops.c | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
index cfed3c2..5d98eec 100644
--- a/gcc/tree-cfg.c
+++ b/gcc/tree-cfg.c
@@ -5783,7 +5783,7 @@ gimple_split_block_before_cond_jump (basic_block bb)
   if (gimple_code (last) != GIMPLE_COND
       && gimple_code (last) != GIMPLE_SWITCH)
     return NULL;
-  gsi_prev_nondebug (&gsi);
+  gsi_prev (&gsi);
   split_point = gsi_stmt (gsi);
   return split_block (bb, split_point)->dest;
 }
diff --git a/gcc/tree-parloops.c b/gcc/tree-parloops.c
index 6c85634..3d41275 100644
--- a/gcc/tree-parloops.c
+++ b/gcc/tree-parloops.c
@@ -1737,6 +1737,8 @@ transform_to_exit_first_loop_alt (struct loop *loop,
   /* Recalculate dominance info.  */
   free_dominance_info (CDI_DOMINATORS);
   calculate_dominance_info (CDI_DOMINATORS);
+
+  checking_verify_ssa (true, true);
 }
 
 /* Tries to moves the exit condition of LOOP to the beginning of its header
-- 
1.9.1