[PATCH] Fix PR84986

[Richard Biener <rguenther@suse.de>]

With the x86 vectorizer cost-model rewrite we ended up costing
scalar conversions as nothing.  After my patch using the proper
target cost estimates for the scalar version this now exposes
underestimating scalar cost and thus no longer vectorizing
the testcase in this PR.  This fix is to restrict zero-costing
to sign-conversions, all other conversions are possibly
value-changing.  I guess some zero-extensions are free as well
but I didn't want to get too fancy as I'm not sure about
QImode -> SImode conversions for example since whether
that's free (can just use %eax instead of %ax) likely depends on 
context.

Bootstrap and regtest running on x86_64-unknown-linux-gnu.

OK?

Thanks,
Richard.

2018-03-20  Richard Biener  <rguenther@suse.de>

	PR target/84986
	* config/i386/i386.c (ix86_add_stmt_cost): Only cost
	sign-conversions as zero, fall back to standard scalar_stmt
	cost for the rest.

	* gcc.dg/vect/costmodel/x86_64/costmodel-pr84986.c: New testcase.

Index: gcc/config/i386/i386.c
===================================================================
--- gcc/config/i386/i386.c	(revision 258674)
+++ gcc/config/i386/i386.c	(working copy)
@@ -50462,7 +50462,11 @@ ix86_add_stmt_cost (void *data, int coun
 	  }
 	  break;
 	case NOP_EXPR:
-	  stmt_cost = 0;
+	  /* Only sign-conversions are free.  */
+	  if (tree_nop_conversion_p
+	        (TREE_TYPE (gimple_assign_lhs (stmt_info->stmt)),
+		 TREE_TYPE (gimple_assign_rhs1 (stmt_info->stmt))))
+	    stmt_cost = 0;
 	  break;
 
 	case BIT_IOR_EXPR:
Index: gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr84986.c
===================================================================
--- gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr84986.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/vect/costmodel/x86_64/costmodel-pr84986.c	(working copy)
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target vect_long } */
+
+int N;
+long fn1(void) {
+  short i;
+  long a;
+  i = a = 0;
+  while (i < N)
+    a -= i++;
+  return a;
+}
+
+/* { dg-final { scan-tree-dump "vectorized 1 loops" "vect" } } */