Re: [PATCH][simplify-rtx] PR 65235: Calculate element size correctly when simplifying (vec_select (vec_concat (const_int) (...)) [...])

[Marc Glisse]

On Wed, 4 Mar 2015, Kyrill Tkachov wrote:

Hi all,
This patch fixes PR rtl-optimization 65235.
As mentioned in bugzilla:
Combine tries to combine:
(insn 72 71 73 2 (set (reg:V2DI 117 [ D.18177 ])
        (vec_concat:V2DI (reg:DI 176 [ D.18179 ])
          (reg:DI 114 [ D.18168 ])))
     (expr_list:REG_DEAD (reg:DI 176 [ D.18179 ])
     (expr_list:REG_DEAD (reg:DI 114 [ D.18168 ])

and

(insn 104 102 105 2 (set (reg:DI 193 [ D.18168 ])
        (vec_select:DI (reg:V2DI 117 [ D.18177 ])
            (parallel [
                    (const_int 0 [0])
                ])))
     (expr_list:REG_DEAD (reg:V2DI 117 [ D.18177 ])
        (nil)))

but ends up generating:
(set (reg:DI 193 [ D.18168 ])
   (reg:DI 114 [ D.18168 ]))


that is, it gets the wrong element of the vec_concat.

The problem is that in simplify-rtx it tries to get the size of the 
first element, compare the requested offset against it and pick the 
second element if the offset is greater than that size. If the first 
element is a const_int, the size is 0, so it will always pick the second 
part.

I am a bit surprised that the first element is a const_int and not reg 
176, maybe that's why it doesn't reproduce on other platforms? Not that it 
really matters.

The patch fixes that by calculating the size of the first element by 
taking the size of the outer mode and subtracting the size of the second 
element.

I wonder if we should admit that vector sizes are always a power of 2 and 
use half the size of the outer mode?

I've added an assert to make sure that the second element is not also a 
const_int, as a vec_concat of const_ints doesn't make sense as far as I 
can see.

--
Marc Glisse