On 12/10/25 8:11 AM, Kishan Parmar wrote:
Hello,
The expression (A ^ B) & C ^ B is the canonical GIMPLE form because it
optimizes for instruction count on generic targets. However, for
targets that support ANDN (like PowerPC andc), the equivalent
(A & C) | (B & ~C) form is preferable because it reduces register
dependency chain.
Currently, GCC generates the XOR form which creates a serial chain with
two dependencies:
xor 4,3,4
and 4,4,5
xor 3,4,3
With this patch, using IFN_BIT_ANDN, we generate the IOR form. This
allows the two bitwise operations to execute independently, reducing
the path to a single dependency for the final instruction:
andc 3,3,5
and 2,4,5
or 3,2,3
This patch fixes PR90323 and PR122431. Tested on powerpc64le-linux-gnu
with no regressions.
Thanks,
Kishan
2025-12-10 Kishan Parmar <[email protected]>
gcc/ChangeLog:
PR tree-optimization/122431 target/90323
* config/rs6000/rs6000.md (andn<mode>3): New define_expand andn for
scalar types.
* match.pd: Add late simplification to convert to (A & C) | (B & ~C)
form if target support ANDN optab.
FWIW, this ought to help RISC-V as well once we fix the port to use the
proper name.
Once Andrew is happy, this is OK.
jeff
