change get_best_mode args int -> HOST_WIDE_INT [PR121264]

The following testcase is miscompiled, because byte 0x20000000
is bit 0x100000000 and ifcombine incorrectly combines the two loads
into a BIT_FIELD_REF even when they are very far away.
The problem is that gimple-fold.cc ifcombine uses get_best_mode heavily,
and that function has just int bitsize and int bitpos arguments, so
when called e.g. with
  if (get_best_mode (end_bit - first_bit, first_bit, 0, ll_end_region,
                     ll_align, BITS_PER_WORD, volatilep, &lnmode))
where end_bit - first_bit doesn't fit into int, it is silently truncated.
If there was just a single problematic get_best_mode call, I would probably
just check for overflows in the caller, but there are many.
And the two arguments are used solely as arguments to
bit_field_mode_iterator constructor which has HOST_WIDE_INT arguments,
so I think the easiest fix is just make the get_best_mode arguments
also HOST_WIDE_INT.

2025-07-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/121264
	* machmode.h (get_best_mode): Change type of first 2 arguments
	from int to HOST_WIDE_INT.
	* stor-layout.cc (get_best_mode): Likewise.

	* gcc.dg/tree-ssa/pr121264.c: New test.

diff --git a/gcc/machmode.h b/gcc/machmode.h
index 467681d9d2f39d0f1827221ef49be416b45b7449..2f2d349be29c9b3e4df46dbf52e3e3afdeb234a5 100644 (file)
--- a/gcc/machmode.h
+++ b/gcc/machmode.h
@@ -958,7 +958,8 @@ private:
 
 /* Find the best mode to use to access a bit field.  */
 
-extern bool get_best_mode (int, int, poly_uint64, poly_uint64, unsigned int,
+extern bool get_best_mode (HOST_WIDE_INT, HOST_WIDE_INT,
+                          poly_uint64, poly_uint64, unsigned int,
                           unsigned HOST_WIDE_INT, bool, scalar_int_mode *);
 
 /* Determine alignment, 1<=result<=BIGGEST_ALIGNMENT.  */

diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index 12071c96ca79f96ea571038909570bd4f756de60..63e830a4d2e2db6e891a6bb5ecc08e8d095ad3c3 100644 (file)
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -3167,7 +3167,7 @@ bit_field_mode_iterator::prefer_smaller_modes ()
    decide which of the above modes should be used.  */
 
 bool
-get_best_mode (int bitsize, int bitpos,
+get_best_mode (HOST_WIDE_INT bitsize, HOST_WIDE_INT bitpos,
               poly_uint64 bitregion_start, poly_uint64 bitregion_end,
               unsigned int align,
               unsigned HOST_WIDE_INT largest_mode_bitsize, bool volatilep,

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr121264.c b/gcc/testsuite/gcc.dg/tree-ssa/pr121264.c
new file mode 100644 (file)
index 0000000..bd5acc0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr121264.c
@@ -0,0 +1,12 @@
+/* PR tree-optimization/121264 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump " \\\| " "optimized" } } */
+
+struct A { char b; char c[0x20000010]; } a;
+
+int
+foo ()
+{
+  return a.c[0x20000000] || a.c[1];
+}