(true_dependence): An unchanging read is guaranteed independent of a store only…

(true_dependence): An unchanging read is guaranteed independent of a store only if the store is not unchanging.

(true_dependence): An unchanging read is guaranteed
independent of a store only if the store is not unchanging.
(anti_dependence): Added comment about unchanging reads.
(sched_analyze_2): Don't ignore unchanging reads, they may be
dependent on unchanging writes.

From-SVN: r1777

gcc/sched.c

@@ -653,7 +653,14 @@ true_dependence (mem, x)
      rtx mem;
      rtx x;
 {
-  if (RTX_UNCHANGING_P (x))
+  /* If X is an unchanging read, then it can't possibly conflict with any
+     non-unchanging store.  It may conflict with an unchanging write though,
+     because there may be a single store to this address to initialize it.
+     Just fall through to the code below to resolve the case where we have
+     both an unchanging read and an unchanging write.  This won't handle all
+     cases optimally, but the possible performance loss should be
+     negligible.  */
+  if (RTX_UNCHANGING_P (x) && ! RTX_UNCHANGING_P (mem))
     return 0;
 
   return ((MEM_VOLATILE_P (x) && MEM_VOLATILE_P (mem))
@@ -672,6 +679,9 @@ anti_dependence (mem, x)
      rtx mem;
      rtx x;
 {
+  /* If MEM is an unchanging read, then it can't possibly conflict with
+     the store to X, because there is at most one store to MEM, and it must
+     have occured somewhere before MEM.  */
   if (RTX_UNCHANGING_P (mem))
     return 0;
 
@@ -1389,11 +1399,6 @@ sched_analyze_2 (x, insn)
       {
 	/* Reading memory.  */
 
-	/* Don't create a dependence for memory references which are known to
-	   be unchanging, such as constant pool accesses.  These will never
-	   conflict with any other memory access.  */
-	if (RTX_UNCHANGING_P (x) == 0)
-	  {
 	rtx pending, pending_mem;
 
 	pending = pending_read_insns;
@@ -1428,7 +1433,7 @@ sched_analyze_2 (x, insn)
 	   this insn may be followed by a write.  */
 	add_insn_mem_dependence (&pending_read_insns, &pending_read_mems,
 				 insn, x);
-	  }
+
 	/* Take advantage of tail recursion here.  */
 	sched_analyze_2 (XEXP (x, 0), insn);
 	return;