On 12/7/25 1:44 PM, Florian Weimer wrote:
* Jeff Law:
This is Shreya's work except for the SH testcase which I added after
realizing her work would also fix the testcases for that port. I
bootstrapped and regression tested this on sh4-linux-gnu, x86_64 &
risc-v. It also was tested across all the embedded targets in my tester
without regressions.
--
We are extracting two single-bit bitfields from a structure and
determining whether they both have the value 0 or if at least one bit is
set. This has been generating poor code:
> lw a5,0(a0)
> bexti a0,a5,1
> bexti a5,a5,2
> or a0,a0,a5
> ret
We address this as a simplification problem and optimize this using an
andi of the original value and a mask with just the desired bits set,
followed by a snez. This results in a 1 if any of those bits are set or
0 if none.
For cases where we want to extract three or more single-bit bitfields,
we build on the previous case. We take the result of the 2-bitfield
case, extract the mask, update it to include the new single-bit
bitfield, and again perform an andi + snez.
In our new testfile, we scan to ensure we do not see a bexti or an or
instruction, and that we have the correct assembly for both two and
three single-bit bitfield cases: lw + andi + snez + ret.
We still have horrible code generation for this on x86-64.
From the bug report:
[ ... ]
I know. That's part of why the bug is staying open.
Is there no generic infrastructure that could handle this?
That was the hope of doing it in simplify-rtx since that's a common low
level simplifier module. But's not necessarily sufficient.
match.pd isn't great for this as we'd need to see a single load covering
the two fields within the structure, that's not trivially exposed until RTL.
And the actual formation in simplify-rtx can look different on different
targets and needs to match something the x86 port defines. It looks
like x86 needs a pattern like this;
Failed to match this instruction:
(parallel [
(set (reg:QI 102 [ _5 ])
(ne:QI (and:QI (reg:QI 106 [ *s_4(D) ])
(const_int 6 [0x6]))
(const_int 0 [0])))
(clobber (reg:CC 17 flags))
])
Jeff
