On 8/13/2026 1:36 AM, Artemiy Volkov wrote:
This patch adds a simplification rule for expressions of the form
((X >> C1) & C2) << C3, which extract some bits from X at position C1,
perform an "and" with a mask which is normally just 2^N - 1, then shift
the result left by C3. The transformation is limited to cases where X
is unsigned, has its precision equal to its width and where C1 and C3
are smaller than the precision of X (the last condition could probably
be just assumed but I wasn't sure so decided to play it safe).
When all of the above conditions hold, the expression is folded into
either: (a) (X >> (C1 - C3)) & (C2 << C3) when C1 >= C3, or (b)
(X << (C3 - C1)) & (C2 << C3) when C1 < C3. Additional care is required
to preserve the leading zeros formed by the X >> C1 operation in the
original expression; to handle this, we clear the leading bits of the
mask operand as a preliminary step.
The corner case where C1 is one less the precision of X is handled
elsewhere (and is folded to just (X >> C1) << C3 as long as the LSB of
C2 isn't 0.)
On aarch64, this results in:
lsr x0, x0, 16
and w0, w0, 130816
being emitted instead of:
lsr x1, x0, 32
lsr x0, x0, 24
ubfiz w1, w1, 16, 1
ubfiz w0, w0, 8, 8
orr w0, w1, w0
for the expression "((x >> 32) & 1) << 16) | (x >> 24) & 0xff) << 8)".
A couple of new testcases added, with some focus on the case where X is
shifted too far to the right as described above.
Survives bootstrap and regtest on aarch64-linux-gnu and x86_64-linux-gnu.
gcc/ChangeLog:
* match.pd: New rule to fold ((X >> C1) & C2) << C3.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/match-bit-extract-shift.c: New test.
Interesting you should start looking at this. I just pointed Daniel at
a closely related problem.
In particular should we recognize the (x >> C) & 2^n-1 as a
BIT_FIELD_REF. Doing so for the single bit case would help pr32648 on
targets that have single bit extraction/manipulation like RISC-V.
I vaguely recall concerns that we didn't want to recognize or
canonicalize to BIT_FIELD_REF in the past, but the details escape me.
Might as well get that discussion started since if we target
BIT_FIELD_REF it's going to mean this patch would need further
adjustment. I seem to think it was Richi or Andrea that held this
position, but far from 100% certain on that.
jeff
