[PATCH] Fix PR91812

[Richard Biener]

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.

Richard.

2019-09-19  Richard Biener  <rguent...@suse.de>

        PR tree-optimization/91812
        * tree-ssa-phiprop.c (propagate_with_phi): Do not replace
        volatile loads.

        * gcc.dg/torture/pr91812.c: New testcase.

Index: gcc/tree-ssa-phiprop.c
===================================================================
--- gcc/tree-ssa-phiprop.c      (revision 275698)
+++ gcc/tree-ssa-phiprop.c      (working copy)
@@ -338,8 +338,15 @@ propagate_with_phi (basic_block bb, gphi
            && (!type
                || types_compatible_p
                     (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
-           /* We cannot replace a load that may throw or is volatile.  */
-           && !stmt_can_throw_internal (cfun, use_stmt)))
+           /* We cannot replace a load that may throw or is volatile.
+              For volatiles the transform can change the number of
+              executions if the load is inside a loop but the address
+              computations outside (PR91812).  We could relax this
+              if we guard against that appropriately.  For loads that can
+              throw we could relax things if the moved loads all are
+              known to not throw.  */
+           && !stmt_can_throw_internal (cfun, use_stmt)
+           && !gimple_has_volatile_ops (use_stmt)))
        continue;
 
       /* Check if we can move the loads.  The def stmt of the virtual use
Index: gcc/testsuite/gcc.dg/torture/pr91812.c
===================================================================
--- gcc/testsuite/gcc.dg/torture/pr91812.c      (nonexistent)
+++ gcc/testsuite/gcc.dg/torture/pr91812.c      (working copy)
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-fdump-tree-optimized-blocks" } */
+
+unsigned register1;
+unsigned register2;
+
+void busy_wait_for_register (int x)
+{
+  volatile unsigned* ptr;
+  switch(x) {
+    case 0x1111:
+    ptr = &register1;
+    break;
+
+    case 0x2222:
+    ptr = &register2;
+    break;
+
+    default:
+    return;
+  }
+  while (*ptr) {}
+}
+
+/* { dg-final { scan-tree-dump "loop depth 1" "optimized" } } */