[llvm-dev] Wide load/store optimization question

[Peter Bel via llvm-dev]

Hi,

I've looked through both AMDGPU and Sparc backends, and it seems they also
do not perform the thing I want to make. The only backend which is doing it
is AArch64, but it doesn't have reg constraints.
So, just with an example. I have the following C code:

void test()
{
  int a = 1; int b = 2; int c = 3; int d = 4;
  a++; b++; c++; d++;
}

Without any frontend optimization is compiles to the following IR.

define void @test(i32* %z) #0 {
  %1 = alloca i32*, align 4
  %a = alloca i32, align 4
  %b = alloca i32, align 4
  %c = alloca i32, align 4
  %d = alloca i32, align 4
  store i32* %z, i32** %1, align 4
  store i32 1, i32* %a, align 4
  store i32 2, i32* %b, align 4
  store i32 3, i32* %c, align 4
  store i32 4, i32* %d, align 4
  %2 = load i32, i32* %a, align 4
  %3 = add nsw i32 %2, 1
  store i32 %3, i32* %a, align 4
  %4 = load i32, i32* %b, align 4
  %5 = add nsw i32 %4, 1
  store i32 %5, i32* %b, align 4
  .....
}

Which produces the following asm code.

        mov     r2, #1
        str     r2, [fp, #-2]
        mov     r3, #2
        mov     r2, #3
        str     r3, [fp, #-3]
        str     r2, [fp, #-4]
        mov     r3, #4
        ldr     r2, [fp, #-2]
        str     r3, [fp, #-5]
        .....

What I want to do is to merge neighboring stores and loads. For example
        mov     r3, #2
        mov     r2, #3
        str     r3, [fp, #-5]
        str     r2, [fp, #-4]
Can be converted to
        mov     r3, #2
        mov     r2, #3
        strd    r2, [fp, #-4]
But the main problem is that the offset for r3 in the snippet above was -3,
not -5.

Currently, i'm doing the following. During the pre-RA i'm creating a
REG_SEQUENCE with the target class, assigning vregs in question as its
subregs, and create a load/store inst for the sequence with mem references
merged.
It solves the register constraint problem, but the frame allocation problem
still exists. Probably I'll need to use fixed stack objects and manually
pre-allocate the frame, which i really don't want to do as it can break
some other passes.

Petr


On Sat, Jun 17, 2017 at 10:31 AM, 陳韋任 <chenwj.cs97g at g2.nctu.edu.tw> wrote:

> That question makes no sense.
>> - Every virtual register has a register class assigned.
>> - You can construct special register classes that represent register
>> tuples so that when the allocator chooses an entry from that register class
>> it really has choosen a tuple of machine registers (even though it looks
>> like a single register with funny aliasing as far as llvm codegen is
>> concerned).
>>
>
> ​And we still have to lower load i64 to load v2i32, right?​
>
> --
> Wei-Ren Chen (陳韋任)
> Homepage: https://people.cs.nctu.edu.tw/~chenwj
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170628/76585ca2/attachment.html>