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Improve do_store_flag for single bit comparison against 0

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While working something else, I noticed we could improve
the following function code generation:
```
unsigned f(unsigned t)
{
  if (t & ~(1<<30)) __builtin_unreachable();
  return t != 0;
}
```
Right know we just emit a comparison against 0 instead
of just a shift right by 30.
There is code in do_store_flag which already optimizes
`(t & 1<<30) != 0` to `(t >> 30) & 1` (using bit extraction if available).
This patch extends it to handle the case where we know t has a nonzero
of just one bit set.

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

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* expr.cc (do_store_flag): Extend the one bit checking case
	to handle the case where we don't have an and but rather still
	one bit is known to be non-zero.
This commit is contained in:
Andrew Pinski 2023-05-18 21:38:55 +00:00
parent f66e0a94ad
commit 908e5ab5c1
1 changed files with 20 additions and 5 deletions
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25
gcc/expr.cc
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@ -13162,16 +13162,31 @@ do_store_flag (sepops ops, rtx target, machine_mode mode)
&& integer_zerop (arg1)
&& (TYPE_PRECISION (ops->type) != 1 || TYPE_UNSIGNED (ops->type)))
{
gimple *srcstmt = get_def_for_expr (arg0, BIT_AND_EXPR);
if (srcstmt
&& integer_pow2p (gimple_assign_rhs2 (srcstmt)))
wide_int nz = tree_nonzero_bits (arg0);
if (wi::popcount (nz) == 1)
{
tree op0;
int bitnum;
gimple *srcstmt = get_def_for_expr (arg0, BIT_AND_EXPR);
/* If the defining statement was (x & POW2), then remove the and
as we are going to add it back. */
if (srcstmt
&& integer_pow2p (gimple_assign_rhs2 (srcstmt)))
{
op0 = gimple_assign_rhs1 (srcstmt);
bitnum = tree_log2 (gimple_assign_rhs2 (srcstmt));
}
else
{
op0 = arg0;
bitnum = wi::exact_log2 (nz);
}
enum tree_code tcode = code == NE ? NE_EXPR : EQ_EXPR;
int bitnum = tree_log2 (gimple_assign_rhs2 (srcstmt));
type = lang_hooks.types.type_for_mode (mode, unsignedp);
return expand_single_bit_test (loc, tcode,
gimple_assign_rhs1 (srcstmt),
op0,
bitnum, type, target, mode);
}
}