[PATCH] fold-const: Don't access bit fields with too big mode (PR71310)

[Segher Boessenkool]

On Fri, Jun 10, 2016 at 01:14:16PM -0600, Jeff Law wrote:
> The change itself is fine, and it's approved with a testcase or at least 
> an explanation of why you can't turn either of the tests from the BZ 
> into a testcase in our framework.

This is what I committed:


2016-06-11  Segher Boessenkool  <segher@kernel.crashing.org>

	PR middle-end/71310
	* fold-const.c (optimize_bit_field_compare): Don't try to use
	word_mode unconditionally for reading the bit field, look at
	DECL_BIT_FIELD_REPRESENTATIVE instead.

gcc/testsuite/
	PR middle-end/71310
	* gcc.target/powerpc/pr71310.c: New testcase.

---
 gcc/fold-const.c                           | 13 ++++++++++++-
 gcc/testsuite/gcc.target/powerpc/pr71310.c | 23 +++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/powerpc/pr71310.c

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 5058746..f067001 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -3903,13 +3903,24 @@ optimize_bit_field_compare (location_t loc, enum tree_code code,
        return 0;
    }
 
+  /* Don't use a larger mode for reading the bit field than we will
+     use in other places accessing the bit field.  */
+  machine_mode largest_mode = word_mode;
+  if (TREE_CODE (lhs) == COMPONENT_REF)
+    {
+      tree field = TREE_OPERAND (lhs, 1);
+      tree repr = DECL_BIT_FIELD_REPRESENTATIVE (field);
+      if (repr)
+	largest_mode = DECL_MODE (repr);
+    }
+
   /* See if we can find a mode to refer to this field.  We should be able to,
      but fail if we can't.  */
   nmode = get_best_mode (lbitsize, lbitpos, 0, 0,
 			 const_p ? TYPE_ALIGN (TREE_TYPE (linner))
 			 : MIN (TYPE_ALIGN (TREE_TYPE (linner)),
 				TYPE_ALIGN (TREE_TYPE (rinner))),
-			 word_mode, false);
+			 largest_mode, false);
   if (nmode == VOIDmode)
     return 0;
 
diff --git a/gcc/testsuite/gcc.target/powerpc/pr71310.c b/gcc/testsuite/gcc.target/powerpc/pr71310.c
new file mode 100644
index 0000000..6869a50
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr71310.c
@@ -0,0 +1,23 @@
+/* { dg-do compile { target { powerpc*-*-* } } } */
+/* { dg-options "-O2" } */
+
+/* { dg-final { scan-assembler-not {\mld} } } */
+/* { dg-final { scan-assembler-not {\mlwz} } } */
+/* { dg-final { scan-assembler-times {\mlbz} 2 } } */
+
+struct mmu_gather {
+        long end;
+        int fullmm : 1;
+};
+
+void __tlb_reset_range(struct mmu_gather *p1)
+{
+        if (p1->fullmm)
+                p1->end = 0;
+}
+
+void tlb_gather_mmu(struct mmu_gather *p1)
+{
+        p1->fullmm = 1;
+        __tlb_reset_range(p1);
+}
-- 
1.9.3