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[PATCH] Fix bootstrap miscompare by STV (PR91580)

The following fixes the bootstrap-debug miscompare caused by STV
where we ended up with chain-to-scalar copies just because of
debug uses.  Instead we have to avoid that, eventually substituting
into debug uses or resetting debug stmts when there are reaching
defs from both inside and outside of the chain (since we rename
all in-chain defs).

Bootstrapped on i686-linux-gnu (with a setup previously
reproducing the miscompare).  Bootstrapped on x86_64-unknown-linux-gnu,
testing in progress.

OK for trunk?


2019-08-29  Richard Biener  <>

	PR bootstrap/91580
	* config/i386/i386-features.c (general_scalar_chain::convert_insn):
	Do not emit scalar copies for debug-insns, instead replace
	their uses with the reg copy used in the chain or reset them
	if there is a reaching definition outside of the chain as well.

Index: gcc/config/i386/i386-features.c
--- gcc/config/i386/i386-features.c	(revision 274991)
+++ gcc/config/i386/i386-features.c	(working copy)
@@ -880,18 +880,52 @@ general_scalar_chain::convert_op (rtx *o
 general_scalar_chain::convert_insn (rtx_insn *insn)
-  /* Generate copies for out-of-chain uses of defs.  */
+  /* Generate copies for out-of-chain uses of defs and adjust debug uses.  */
   for (df_ref ref = DF_INSN_DEFS (insn); ref; ref = DF_REF_NEXT_LOC (ref))
     if (bitmap_bit_p (defs_conv, DF_REF_REGNO (ref)))
 	df_link *use;
 	for (use = DF_REF_CHAIN (ref); use; use = use->next)
-	  if (DF_REF_REG_MEM_P (use->ref)
-	      || !bitmap_bit_p (insns, DF_REF_INSN_UID (use->ref)))
+	  if (NONDEBUG_INSN_P (DF_REF_INSN (use->ref))
+	      && (DF_REF_REG_MEM_P (use->ref)
+		  || !bitmap_bit_p (insns, DF_REF_INSN_UID (use->ref))))
 	if (use)
 	  convert_reg (insn, DF_REF_REG (ref),
 		       *defs_map.get (regno_reg_rtx [DF_REF_REGNO (ref)]));
+	else
+	  {
+	    /* If we generated a scalar copy we can leave debug-insns
+	       as-is, if not, we have to adjust them.  */
+	    auto_vec<rtx_insn *, 5> to_reset_debug_insns;
+	    for (use = DF_REF_CHAIN (ref); use; use = use->next)
+	      if (DEBUG_INSN_P (DF_REF_INSN (use->ref)))
+		{
+		  rtx_insn *debug_insn = DF_REF_INSN (use->ref);
+		  /* If there's a reaching definition outside of the
+		     chain we have to reset.  */
+		  df_link *def;
+		  for (def = DF_REF_CHAIN (use->ref); def; def = def->next)
+		    if (!bitmap_bit_p (insns, DF_REF_INSN_UID (def->ref)))
+		      break;
+		  if (def)
+		    to_reset_debug_insns.safe_push (debug_insn);
+		  else
+		    {
+		      *DF_REF_REAL_LOC (use->ref)
+			= *defs_map.get (regno_reg_rtx [DF_REF_REGNO (ref)]);
+		      df_insn_rescan (debug_insn);
+		    }
+		}
+	    /* Have to do the reset outside of the DF_CHAIN walk to not
+	       disrupt it.  */
+	    while (!to_reset_debug_insns.is_empty ())
+	      {
+		rtx_insn *debug_insn = to_reset_debug_insns.pop ();
+		INSN_VAR_LOCATION_LOC (debug_insn) = gen_rtx_UNKNOWN_VAR_LOC ();
+		df_insn_rescan_debug_internal (debug_insn);
+	      }
+	  }
   /* Replace uses in this insn with the defs we use in the chain.  */

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