Re: [PATCH 2/2] Improve do_store_flag for comparing single bit against that bit

[Jeff Law via Gcc-patches]

On 5/18/23 20:14, Andrew Pinski via Gcc-patches wrote:

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.

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.
This looks like it can/should go forward independently of 1/2 and 
touches on my earlier comment about using bit extractions  ;-)


So OK by me.

jeff

[PATCH 2/2] Improve do_store_flag for comparing single bit against that bit

[Andrew Pinski via Gcc-patches]

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.

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.
---
 gcc/expr.cc | 13 +
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 91528e734e7..a4628c51c0c 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -13080,12 +13080,15 @@ do_store_flag (sepops ops, rtx target, machine_mode 
mode)
  so we just call into the folder and expand its result.  */
 
   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;
  tree op1;
@@ -13103,11 +13106,13 @@ do_store_flag (sepops ops, rtx target, machine_mode 
mode)
  op0 = arg0;
  op1 = wide_int_to_tree (TREE_TYPE (op0), 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);
  tree temp = fold_build2_loc (loc, BIT_AND_EXPR, TREE_TYPE (op0),
   op0, op1);
- temp = fold_single_bit_test (loc, tcode, temp, arg1, type);
+ temp = fold_single_bit_test (loc, tcode, temp, build_zero_cst (type), 
type);
  if (temp)
return expand_expr (temp, target, VOIDmode, EXPAND_NORMAL);
}
-- 
2.31.1