Fix over-widening handling of COND_EXPRs (PR 86749)

[Richard Sandiford]

This PR is a wrong-code bug caused by the over-widening support.
The minimum input precisions for a COND_EXPR are supposed to apply
only to the "then" and "else" values, but here we were applying
them to the operands of a nested COND_EXPR comparison instead.

Tested on aarch64-linux-gnu (with and without SVE), aarch64_be-elf
and x86_64-linux-gnu.  OK to install?

Richard


2018-08-01  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86749
	* tree-vect-patterns.c (vect_determine_min_output_precision_1):
	If the lhs is used in a COND_EXPR, check that it is being used
	as the "then" or "else" value.

gcc/testsuite/
	PR tree-optimization/86749
	* gcc.dg/vect/pr86749.c: New test.

Index: gcc/tree-vect-patterns.c
===================================================================
--- gcc/tree-vect-patterns.c	2018-07-31 15:26:37.102461003 +0100
+++ gcc/tree-vect-patterns.c	2018-08-01 10:12:45.865913009 +0100
@@ -4399,6 +4399,14 @@ vect_determine_min_output_precision_1 (s
       stmt_vec_info use_stmt_info = vinfo->lookup_stmt (use_stmt);
       if (!use_stmt_info || !use_stmt_info->min_input_precision)
 	return false;
+      /* The input precision recorded for COND_EXPRs applies only to the
+	 "then" and "else" values.  */
+      gassign *assign = dyn_cast <gassign *> (stmt_info->stmt);
+      if (assign
+	  && gimple_assign_rhs_code (assign) == COND_EXPR
+	  && use->use != gimple_assign_rhs2_ptr (assign)
+	  && use->use != gimple_assign_rhs3_ptr (assign))
+	return false;
       precision = MAX (precision, use_stmt_info->min_input_precision);
     }
 
Index: gcc/testsuite/gcc.dg/vect/pr86749.c
===================================================================
--- /dev/null	2018-07-26 10:26:13.137955424 +0100
+++ gcc/testsuite/gcc.dg/vect/pr86749.c	2018-08-01 10:12:45.865913009 +0100
@@ -0,0 +1,26 @@
+/* { dg-additional-options "-O3" } */
+
+#include "tree-vect.h"
+
+short a, b, f, g;
+int c = 4, d, e = -1L;
+long h = 4;
+
+int
+main ()
+{
+  check_vect ();
+
+  long i;
+  for (; d <= 55; d++)
+    {
+      g = c >= 2 ? 0 : b << c;
+      f = g - a;
+      i = (f ^ 9223372036854775807) < 0 ? f : h;
+      e &= i;
+    }
+  if (e != 4)
+    __builtin_abort ();
+
+  return 0;
+}