http://gcc.gnu.org/PR20973

"gcc 4 (about RC1) miscompiles khtml, in fact something in CSS, which basically 

leads to all websites being misrendered.  I can't easily reduce the testcase, 

but have applied the whole preprocessed source of css/cssstyleselector.ii. 

It is to be compiled with g++ -O2 -fPIC -march=i586 -mtune=i686 

-fno-exceptions.  A more detailed analysis will follow, as we've found out 

some things already."

---

Subject: Bug 20973

CVSROOT:	/cvs/gcc

Module name:	gcc

Branch: 	gcc-4_0-branch

Changes by:	matz@gcc.gnu.org	2005-04-22 17:30:21

Modified files:

	gcc            : ChangeLog reload.c 

Log message:

	PR middle-end/20973

	* reload.c (push_reload, find_dummy_reload): Check for uninitialized

	pseudos.

Patches:

http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=2.7592.2.177&r2=2.7592.2.178

http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/reload.c.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=1.268&r2=1.268.2.1

---

===================================================================

RCS file: /cvs/gcc/gcc/gcc/reload.c,v

retrieving revision 1.268

retrieving revision 1.268.2.1

diff -u -r1.268 -r1.268.2.1

--- gcc/gcc/reload.c	2005/02/24 22:06:06	1.268

+++ gcc/gcc/reload.c	2005/04/22 17:30:15	1.268.2.1

@@ -1520,7 +1520,7 @@

      But if there is no spilling in this block, that is OK.

      An explicitly used hard reg cannot be a spill reg.  */

-  if (rld[i].reg_rtx == 0 && in != 0)

+  if (rld[i].reg_rtx == 0 && in != 0 && hard_regs_live_known)

     {

       rtx note;

       int regno;

@@ -1534,6 +1534,11 @@

 	    && REG_P (XEXP (note, 0))

 	    && (regno = REGNO (XEXP (note, 0))) < FIRST_PSEUDO_REGISTER

 	    && reg_mentioned_p (XEXP (note, 0), in)

+	    /* Check that we don't use a hardreg for an uninitialized

+	       pseudo.  See also find_dummy_reload().  */

+	    && (ORIGINAL_REGNO (XEXP (note, 0)) < FIRST_PSEUDO_REGISTER

+		|| ! bitmap_bit_p (ENTRY_BLOCK_PTR->global_live_at_end,

+				   ORIGINAL_REGNO (XEXP (note, 0))))

 	    && ! refers_to_regno_for_reload_p (regno,

 					       (regno

 						+ hard_regno_nregs[regno]

@@ -1997,7 +2002,17 @@

 				is a subreg, and in that case, out

 				has a real mode.  */

 			     (GET_MODE (out) != VOIDmode

-			      ? GET_MODE (out) : outmode)))

+			      ? GET_MODE (out) : outmode))

+        /* But only do all this if we can be sure, that this input

+           operand doesn't correspond with an uninitialized pseudoreg.

+           global can assign some hardreg to it, which is the same as

+	   a different pseudo also currently live (as it can ignore the

+	   conflict).  So we never must introduce writes to such hardregs,

+	   as they would clobber the other live pseudo using the same.

+	   See also PR20973.  */

+      && (ORIGINAL_REGNO (in) < FIRST_PSEUDO_REGISTER

+          || ! bitmap_bit_p (ENTRY_BLOCK_PTR->global_live_at_end,

+			     ORIGINAL_REGNO (in))))

     {

       unsigned int regno = REGNO (in) + in_offset;

       unsigned int nwords = hard_regno_nregs[regno][inmode];