Re: shift/extract SHIFT_COUNT_TRUNCATED combine bug

[Jeff Law <law@redhat.com>]

On 04/08/14 14:07, Mike Stump wrote:
> Something broke in the compiler to cause combine to incorrectly optimize:
>
> (insn 12 11 13 3 (set (reg:SI 604 [ D.6102 ])
>          (lshiftrt:SI (subreg/s/u:SI (reg/v:DI 601 [ x ]) 0)
>              (reg:SI 602 [ D.6103 ]))) t.c:47 4436 {lshrsi3}
>       (expr_list:REG_DEAD (reg:SI 602 [ D.6103 ])
>          (nil)))
> (insn 13 12 14 3 (set (reg:SI 605)
>          (and:SI (reg:SI 604 [ D.6102 ])
>              (const_int 1 [0x1]))) t.c:47 3658 {andsi3}
>       (expr_list:REG_DEAD (reg:SI 604 [ D.6102 ])
>          (nil)))
> (insn 14 13 15 3 (set (reg:DI 599 [ D.6102 ])
>          (zero_extend:DI (reg:SI 605))) t.c:47 4616 {zero_extendsidi2}
>       (expr_list:REG_DEAD (reg:SI 605)
>          (nil)))
>
> into:
>
> (insn 11 10 12 3 (set (reg:SI 602 [ D.6103 ])
>          (not:SI (subreg:SI (reg:DI 595 [ D.6102 ]) 0))) t.c:47 3732 {one_cmplsi2}
>       (expr_list:REG_DEAD (reg:DI 595 [ D.6102 ])
>          (nil)))
> (note 12 11 13 3 NOTE_INSN_DELETED)
> (note 13 12 14 3 NOTE_INSN_DELETED)
> (insn 14 13 15 3 (set (reg:DI 599 [ D.6102 ])
>          (zero_extract:DI (reg/v:DI 601 [ x ])
>              (const_int 1 [0x1])
>              (reg:SI 602 [ D.6103 ]))) t.c:47 4668 {c2_extzvdi}
>       (expr_list:REG_DEAD (reg:SI 602 [ D.6103 ])
>          (nil)))
>
> This shows up in:
>
>    FAIL: gcc.c-torture/execute/builtin-bitops-1.c execution,  -Og -g
>
> for me.
>
> diff --git a/gcc/combine.c b/gcc/combine.c
> index 708691f..c1f50ff 100644
> --- a/gcc/combine.c
> +++ b/gcc/combine.c
> @@ -7245,6 +7245,18 @@ make_extraction (enum machine_mode mode, rtx inner, HOST_WIDE_INT pos,
>         extraction_mode = insn.field_mode;
>       }
>
> +  /* On a SHIFT_COUNT_TRUNCATED machine, we can't promote the mode of
> +     the extract to a larger size on a variable extract, as previously
> +     the position might have been optimized to change a bit of the
> +     index of the starting bit that would have been ignored before,
> +     but, with a larger mode, will then not be.  If we wanted to do
> +     this, we'd have to mask out those bits or prove that those bits
> +     are 0.  */
> +  if (SHIFT_COUNT_TRUNCATED
> +      && pos_rtx
> +      && GET_MODE_BITSIZE (extraction_mode) > GET_MODE_BITSIZE (mode))
> +    extraction_mode = mode;
> +
>     /* Never narrow an object, since that might not be safe.  */
>
>     if (mode != VOIDmode
>
> is sufficient to never widen variable extracts on SHIFT_COUNT_TRUNCATED machines.  So, the question is, how did people expect this to work?  I didnÂ’t spot what changed recently to cause the bad code-gen.  The optimization of sub into not is ok, despite how funny it looks, because is feeds into extract which we know by SHIFT_COUNT_TRUNCATED is safe.
>
> Is the patch a reasonable way to fix this?
On a SHIFT_COUNT_TRUNCATED target, I don't think it's ever OK to widen a 
shift, variable or constant.

In the case of a variable shift, we could easily have eliminated the 
masking code before or during combine.  For a constant shift amount we 
could have adjusted the constant (see SHIFT_COUNT_TRUNCATED in cse.c)

I think it's just an oversight and it has simply never bit us before.

jeff