On Sun, Sep 20, 2026 at 10:41:20AM -0600, Jeff Law wrote: > > > On 9/6/26 3:37 AM, Artemiy Volkov wrote: > > On Thu, Aug 13, 2026 at 07:36:36AM +0000, 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. > > Hi Jeff, Richard, > > > > Considering the discussion in the main thread seems to have settled on > > recognizing BFRs at the RTL level, could the original patch please be > > considered for review/inclusion? > Agreed.� Let's get this moving again. > > > > 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. > This looks sound to me.� I'll set aside my desire to consider special casing > 2^n-1 cases for C2 (particularly for n=1). > > OK for the trunk.
Pushed after slightly improving the comment, Thanks, Artemiy > > jeff >
