[PATCH, combine] Fix host-specific behavior in simplify_compare_const()

Chung-Ju Wu jasonwucj@gmail.com
Wed Feb 27 16:30:00 GMT 2013

Hi all,

I notice the function simplify_compare_const() in gcc/combine.c presents
different behavior between 32-bit host machine and 64-bit host machine,
causing it to perform different RTL transformation for a 32-bit target.

In the function: simplify_compare_const (enum rtx_code code, rtx op0,
rtx *pop1),
there is a statement "HOST_WIDE_INT const_op = INTVAL (*pop1);".
Although it uses "trunc_int_for_mode()" to make const_op
present the value for target machine mode,
the subsequent condition checking does not filter out the unnecessary bits:

  if (...
      && exact_log2 (const_op) >= 0
      && nonzero_bits (op0, mode) == (unsigned HOST_WIDE_INT) const_op)

For example, given that:

op0: (and:SI (reg/v:SI 54)
             (const_int -2147483648))
*pop1: (const_int -2147483648)

Since mode comes from GET_MODE (op0), we know that mode is SImode.

on 32-bit host machine,

  rtx *pop1: (const_int -2147483648 [0x80000000])
  const_op: 0x80000000

  exact_log2 (const_op): 31
  nonzero_bits (op0, mode)         : 0x80000000
  (unsigned HOST_WIDE_INT) const_op: 0x80000000

on 64-bit host machine,

  rtx *pop1: (const_int -2147483648 [0xffffffff80000000])
  const_op: 0xffffffff80000000

  exact_log2 (const_op): -1
  nonzero_bits (op0, mode)         : 0x0000000080000000
  (unsigned HOST_WIDE_INT) const_op: 0xffffffff80000000

In this case, the condition results on 32-bit and 64-bit host machine
are different.
The result on 64-bit host machine cannot pass the condition test
so that we have no chance to do optimization.

In other words, any 32-bit target with 'need_64bit_hwint=yes' in config.gcc
is not able to have benefit from this optimization because it never
passes the condition test.

My solution is to use GET_MODE_MASK(mode) to filter out all bits not
in target mode.
The following is my patch:


        * gcc/combine.c: Use GET_MODE_MASK() to filter out
        unnecessary bits in simplify_compare_const().

diff --git a/gcc/combine.c b/gcc/combine.c
index 67bd776..8c8cb92 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -10917,8 +10917,8 @@ simplify_compare_const (enum rtx_code code,
rtx op0, rtx *pop1)
       && (code == EQ || code == NE || code == GE || code == GEU
          || code == LT || code == LTU)
       && mode_width <= HOST_BITS_PER_WIDE_INT
-      && exact_log2 (const_op) >= 0
-      && nonzero_bits (op0, mode) == (unsigned HOST_WIDE_INT) const_op)
+      && exact_log2 (const_op & GET_MODE_MASK (mode)) >= 0
+      && nonzero_bits (op0, mode) == (unsigned HOST_WIDE_INT)
(const_op & GET_MODE_MASK (mode)))
       code = (code == EQ || code == GE || code == GEU ? NE : EQ);
       const_op = 0;

Any comment for the patch?


More information about the Gcc-patches mailing list