On Sun, Dec 7, 2025, 12:45 PM Florian Weimer <[email protected]> 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:
>
> typedef struct
> {
> _Bool a : 1;
> _Bool b : 1;
> _Bool c : 1;
> _Bool d : 1;
> unsigned int e : 4;
> } S;
>
>
> _Bool test_00 (S* s)
> {
> return s->b | s->c;
> }
>
> We get this:
>
> test_00:
> movzbl (%rdi), %edx
> movl %edx, %ecx
> movl %edx, %eax
> shrb %cl
> shrb $2, %al
> orl %ecx, %eax
> andl $1, %eax
> ret
>
> This should be something like this instead:
>
> test_00:
> xorl %eax, %eax
> testb $6, (%rdi)
> setne %al
> retq
>
> Is there no generic infrastructure that could handle this?
>
The biggest way of fixing/improving the situation here would be have a
lowering pass for bitfields sometime later say before pre. I had worked on
one before but I never got back to working through the comments on the pass.
Maybe next year I can have some time to mentor someone to implement it and
finish up the pass.
Thanks,
Andrew
>
