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.
---
gcc/expr.cc | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)
diff --git a/gcc/expr.cc b/gcc/expr.cc
index 6849c9627d0..b85e963b57e 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -13155,16 +13155,31 @@ 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);
}
}
--
2.17.1
