This is a regression with -O2 -fno-strict-overflow present on the 4.5 branch.
The compiler generates wrong code because of a problematic transformation done 
by if-convert.  REG_EQUAL notes aren't taken into account to compute liveness 
info so when insns are moved above a conditional branch in the transformation

   (2)
	if (test) goto E; // x not live
	x = big();
	goto L;
	E:
	x = b;
	goto M;

   becomes

	x = b;
	if (test) goto M;
	x = big();
	goto L;

they can change the reaching def of a REG_EQUAL note referring to x in the 
big() part.  A related problem was fixed by Joern in PR rtl-opt/21767, but 
this was for the REG_EQUAL notes attached to the insns being moved.

Fixed by removing the REG_EQUAL notes referring to registers set in the insns 
being moved; these registers are already computed during the transformation.
The patch also fixes inconsistencies in the dumping routines of DF, where uses 
in REG_EQUAL notes were sometimes printed as 'e' and sometimes as 'u'; they 
should now be printed as 'e' by all routines.

Tested on {x86_64,i586}-suse-linux, applied on the mainline and 4.5 branch.


2010-11-16  Eric Botcazou  <ebotcazou@adacore.com>

	PR rtl-optimization/46315
	* rtl.h (remove_reg_equal_equiv_notes_for_regno): Declare.
	* rtlanal.c (remove_reg_equal_equiv_notes_for_regno): New function
	extracted from...
	* dce.c (delete_corresponding_reg_eq_notes): ...here.  Rename into...
	(remove_reg_equal_equiv_notes_for_defs): ...this.
	(delete_unmarked_insns): Adjust to above renaming.
	* ifcvt.c (dead_or_predicable): Remove REG_EQUAL and REG_EQUIV notes
	referring to registers set in the insns being moved, if any.

	* df-core.c (df_ref_dump): New function extracted from...
	(df_refs_chain_dump): ...here.  Call it.
	(df_regs_chain_dump): Likewise.
	* df-problems.c (df_chain_dump): Print 'e' for uses in notes.
	* df-scan.c (df_scan_start_dump): Likewise.  Fix long line.


2010-11-16  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc.dg/pr46315.c: New test.


-- 
Eric Botcazou