[PATCH] PR tree-optimization/78170: Truncate sign-extended padding when encoding bitfields

[Kyrill Tkachov]

Hi all,

In this PR the code writes a -1 to a bitfield of size 17 bits and ends up 
overwriting another bitfields.
The problem is that the intermediate buffer in encode_tree_to_bitpos holding 
the value to merge holds
a 24-bit temporary with -1 written to it i.e. sign-extended to all ones. That 
is how native_encode_expr works.This gets then written to
the final buffer (well, a shifted version of it).

We should instead be truncating the intermediate value to contain zeros in all 
the bits that we don't want.
This is already performed for big-endian, this patch just wires it up for 
little-endian.

Bootstrapped and tested on x86_64.
Ok for trunk?

Thanks,
Kyrill

2016-11-01  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    PR tree-optimization/78170
    * gimple-ssa-store-merging.c (encode_tree_to_bitpos): Truncate padding
    introduced by native_encode_expr on little-endian as well.

2016-11-01  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    PR tree-optimization/78170
    * gcc.c-torture/execute/pr78170.c: New test.

diff --git a/gcc/gimple-ssa-store-merging.c b/gcc/gimple-ssa-store-merging.c
index 5a293d7f30735499aafebeb935b073946eab5691..f82cad35afbc10eea76957d38100acdce137d271 100644
--- a/gcc/gimple-ssa-store-merging.c
+++ b/gcc/gimple-ssa-store-merging.c
@@ -432,13 +432,23 @@ encode_tree_to_bitpos (tree expr, unsigned char *ptr, int bitlen, int bitpos,
      contain a sign bit due to sign-extension).  */
   unsigned int padding
     = byte_size - ROUND_UP (bitlen, BITS_PER_UNIT) / BITS_PER_UNIT - 1;
-  if (BYTES_BIG_ENDIAN)
+  if (padding != 0)
     {
-      tmpbuf += padding;
+      /* On big-endian the padding is at the 'front' so just skip the initial
+	 bytes.  */
+      if (BYTES_BIG_ENDIAN)
+	tmpbuf += padding;
+
       byte_size -= padding;
       if (bitlen % BITS_PER_UNIT != 0)
-	clear_bit_region_be (tmpbuf, BITS_PER_UNIT - 1,
-			     BITS_PER_UNIT - (bitlen % BITS_PER_UNIT));
+	{
+	  if (BYTES_BIG_ENDIAN)
+	    clear_bit_region_be (tmpbuf, BITS_PER_UNIT - 1,
+				 BITS_PER_UNIT - (bitlen % BITS_PER_UNIT));
+	  else
+	    clear_bit_region (tmpbuf, bitlen,
+			      byte_size * BITS_PER_UNIT - bitlen);
+	}
     }
 
   /* Clear the bit region in PTR where the bits from TMPBUF will be
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr78170.c b/gcc/testsuite/gcc.c-torture/execute/pr78170.c
new file mode 100644
index 0000000000000000000000000000000000000000..8ef812ee6accb62db8dd6889d74032a88b784d2c
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr78170.c
@@ -0,0 +1,37 @@
+/* PR tree-optimization/78170.
+   Check that sign-extended store to a bitfield
+   doesn't overwrite other fields.  */
+
+int a, b, d;
+
+struct S0
+{
+  int f0;
+  int f1;
+  int f2;
+  int f3;
+  int f4;
+  int f5:15;
+  int f6:17;
+  int f7:2;
+  int f8:30;
+} c;
+
+void fn1 ()
+{
+  d = b = 1;
+  for (; b; b = a)
+    {
+      struct S0 e = { 0, 0, 0, 0, 0, 0, 1, 0, 1 };
+      c = e;
+      c.f6 = -1;
+    }
+}
+
+int main ()
+{
+  fn1 ();
+  if (c.f7 != 0)
+    __builtin_abort ();
+  return 0;
+}