Improve do_store_flag for comparing single bit against that bit

This is a case which I noticed while working on the previous patch.
Sometimes we end up with `a == CST` instead of comparing against 0.
This happens in the following code:
```
unsigned f(unsigned t)
{
  if (t & ~(1<<30)) __builtin_unreachable();
  t ^= (1<<30);
  return t != 0;
}
```

We should handle the case where the nonzero bits is the same as the
comparison operand.

Changes from v1:
* v2: Updated for the bit extraction changes.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* expr.cc (do_store_flag): Improve for single bit testing
	not against zero but against that single bit.

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 6027e7ffca5af915059143de556760e5de9d1c90..58f5fe76372e57fc681c18777c6fce58369e7f67 100644 (file)
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -13159,12 +13159,15 @@ do_store_flag (sepops ops, rtx target, machine_mode mode)
      than an scc insn even if we have it.  */
 
   if ((code == NE || code == EQ)
-      && integer_zerop (arg1)
+      && (integer_zerop (arg1)
+         || integer_pow2p (arg1))
       && (TYPE_PRECISION (ops->type) != 1 || TYPE_UNSIGNED (ops->type)))
     {
       wide_int nz = tree_nonzero_bits (arg0);
 
-      if (wi::popcount (nz) == 1)
+      if (wi::popcount (nz) == 1
+         && (integer_zerop (arg1)
+             || wi::to_wide (arg1) == nz))
        {
          tree op0;
          int bitnum;
@@ -13182,7 +13185,9 @@ do_store_flag (sepops ops, rtx target, machine_mode mode)
              op0 = arg0;
              bitnum = wi::exact_log2 (nz);
            }
-         enum tree_code tcode = code == NE ? NE_EXPR : EQ_EXPR;
+         enum tree_code tcode = EQ_EXPR;
+         if ((code == NE) ^ !integer_zerop (arg1))
+           tcode = NE_EXPR;
 
          type = lang_hooks.types.type_for_mode (mode, unsignedp);
          return expand_single_bit_test (loc, tcode,