Re: [Patch/combine] PR64304 wrong bitmask passed to force_to_mode in combine_simplify_rtx

[Jeff Law <law at redhat dot com>]

On 01/09/15 06:39, Jiong Wang wrote:
> as reported at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64304
>
> given the following test:
>
> unsigned char byte = 0;
>
> void set_bit(unsigned int bit, unsigned char value) {
>      unsigned char mask = (unsigned char)(1 << (bit & 7));
>      if (!value) {
>          byte &= (unsigned char)~mask;
>      } else {
>          byte |= mask;
>      }
> }
>
> we should generate something like:
>
>    set_bit:
>          and     w0, w0, 7
>          mov     w2, 1
>          lsl     w2, w2, w0
>
> while we are generating
>          mov     w2, 1
>          lsl     w2, w2, w0
>
>
> the necessary "and     w0, w0, 7" deleted wrongly.
>
> that because
>
>    (insn 2 5 3 2 (set (reg/v:SI 82 [ bit ])
>          (reg:SI 0 x0 [ bit ])) bug.c:3 38 {*movsi_aarch64}
>       (expr_list:REG_DEAD (reg:SI 0 x0 [ bit ])
>          (nil)))
>    (insn 7 4 8 2 (set (reg:SI 84 [ D.1482 ])
>          (and:SI (reg/v:SI 82 [ bit ])
>              (const_int 7 [0x7]))) bug.c:4 399 {andsi3}
>       (expr_list:REG_DEAD (reg/v:SI 82 [ bit ])
>          (nil)))
>    (insn 9 8 10 2 (set (reg:SI 85 [ D.1482 ])
>          (ashift:SI (reg:SI 86)
>              (subreg:QI (reg:SI 84 [ D.1482 ]) 0))) bug.c:4 539
> {*aarch64_ashl_sisd_or_int_si3}
>       (expr_list:REG_DEAD (reg:SI 86)
>          (expr_list:REG_DEAD (reg:SI 84 [ D.1482 ])
>              (expr_list:REG_EQUAL (ashift:SI (const_int 1 [0x1])
>                      (subreg:QI (reg:SI 84 [ D.1482 ]) 0))
>                  (nil)))))
>
> are wrongly combined into
>
>    (insn 9 8 10 2 (set (reg:QI 85 [ D.1482 ])
>          (ashift:QI (subreg:QI (reg:SI 86) 0)
>              (reg:QI 0 x0 [ bit ]))) bug.c:4 556 {*ashlqi3_insn}
>       (expr_list:REG_DEAD (reg:SI 0 x0 [ bit ])
>          (expr_list:REG_DEAD (reg:SI 86)
>              (nil))))
>
> thus, the generated assembly is lack of the necessary "and w0, x0, 7".
>
> the root cause is at one place in combine pass, we are passing wrong
> bitmask to force_to_mode.
>
> in this particular case, for QI mode, we should pass (1 << 8 - 1), while
> we are passing (1 << 3 - 1),
> thus the combiner think we only need the lower 3 bits, that X & 7 is
> unnecessary. While for QI mode, we
> want the lower 8 bits. we should remove the exp operator.
>
> this should be a historical bug in combine pass?? while it's only
> triggered for target
> where SHIFT_COUNT_TRUNCATED be true. it's long time hiding mostly
> because x86/arm will
> not trigger this part of code.
>
> bootstrap on x86 and gcc check OK.
> bootstrap on aarch64 and bare-metal regression OK.
> ok for trunk?
>
> gcc/
>    PR64303
>    * combine.c (combine_simplify_rtx): Correct the bitmask passed to
> force_to_mode.
> gcc/testsuite/
>    PR64303
>    * gcc.target/aarch64/pr64304.c: New testcase.
I don't think this is correct.

When I put a breakpoint on the code in question I see the following RTL 
prior to the call to DO_SUBST:

(ashift:QI (const_int 1 [0x1])
    (subreg:QI (and:SI (reg:SI 0 x0 [ bit ])
            (const_int 7 [0x7])) 0))


Note carefully the QImode for the ASHIFT.  That clearly indicates that 
just the low 8 bits are meaningful and on a SHIFT_COUNT_TRUNCATED target 
the masking of the count with 0x7 is redundant as the only valid shift 
counts are 0-7 (again because of the QImode for the ASHIFT).  Thus 
that's equivalent to:


(ashift:QI (const_int 1 [0x1])
    (reg:QI 0 x0 [ bit ]))


Similarly for the case:


(ashift:QI (subreg:QI (reg:SI 85) 0)
    (subreg:QI (and:SI (reg:SI 0 x0 [ bit ])
            (const_int 7 [0x7])) 0))


Again, QImode ASHIFT, so the masking of the shift count is redundant 
resulting in:

(ashift:QI (subreg:QI (reg:SI 85) 0)
    (reg:QI 0 x0 [ bit ]))


I think you need to do some further analysis.  Is it perhaps the case 
that SHIFT_COUNT_TRUNCATED is nonzero when in fact it should be zero?

Jeff