[PATCH] PR optimization/13472

[Eric Botcazou]

Hi,

This is again a problem that shows up only on the 3.3 branch, but is probably 
latent on mainline.  When the testcase is compiled with -O2 -march=i686 
-pedantic, a store to .rodata is emitted, which leads to a segfault at 
runtime.  And, most surprisingly, removing -pedantic makes the problem 
disappear!!

Here's what I wrote in the audit trail:
"The final culprit is the reload pass: it generates the store (although the 
MEM is marked as unchanging) because it thinks the memory location is a 
regular spill slot.  It appears that the reload pass doesn't care about the 
/u flag when recording equivalent memory locations based on the presence of 
REG_EQUIV notes.

But I don't know if this is really a reload problem: maybe reload implicitly
expects full equivalence between objects in pairs that carry REG_EQUIV notes.

In any cases, the dependence upon -pedantic is certainly a bug and is 
actually recognized as such in the code (c-typeck.c:790):

/* Return either DECL or its known constant value (if it has one), but
   return DECL if pedantic or DECL has mode BLKmode.  This is for
   bug-compatibility with the old behavior of decl_constant_value
   (before GCC 3.0); every use of this function is a bug and it should
   be removed before GCC 3.1.  It is not appropriate to use pedantic
   in a way that affects optimization, and BLKmode is probably not the
   right test for avoiding misoptimizations either.  */

static tree
decl_constant_value_for_broken_optimization (decl)
     tree decl;
{
  if (pedantic || DECL_MODE (decl) == BLKmode)
    return decl;
  else
    return decl_constant_value (decl);
}


So I think the fix is to remove decl_constant_value_for_broken_optimization
altogether from the C front-end.  But this is probably not doable on a 
release branch."


I've attached the patch that does this on mainline.  Bootstrapped/regtested
on i586-redhat-linux-gnu (mainline except Ada and libgcj).


2003-12-23  Eric Botcazou  <ebotcazou@libertysurf.fr>

        * c-typeck.c (decl_constant_value_for_broken_optimization): Delete.
	(default_conversion): Call decl_constant_value instead of it.
	(convert_for_assignment): Likewise.
	(digest_init): Likewise.
 

-- 
Eric Botcazou
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