[Bug target/40416] New: unnecessary register spill

[Bug target/40416] New: unnecessary register spill

[carrot at google dot com]

Compile the attached source code with options -O2 -Os -mthumb -fpic, we can get
a unnecessary register spill.


-- 
           Summary: unnecessary register spill
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[carrot at google dot com]

------- Comment #1 from carrot at google dot com  2009-06-11 14:34 -------
Created an attachment (id=17983)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17983&action=view)
test case

The spilling is occurred around the first loop:

        push    {r4, r5, r6, r7, lr}
        sub     sp, sp, #12
        .loc 1 5 0
        str     r2, [sp, #4]          // A
        .loc 1 6 0
        add     r6, r1, r2
        mov     r4, r0
        .loc 1 8 0
        b       .L2
.L5:
        .loc 1 10 0
        mov     r7, #0
        ldrsh   r5, [r4, r7]
        .loc 1 12 0
        cmp     r2, r5
        bge     .L3
        .loc 1 14 0
        ldrb    r7, [r1]
        strb    r7, [r1, r2]
        .loc 1 15 0
        strh    r2, [r4]
        .loc 1 16 0
        lsl     r1, r2, #1
        sub     r2, r5, r2
        strh    r2, [r1, r4]
.L6:
        .loc 1 5 0
        ldr     r5, [sp, #4]     //   B
        lsl     r4, r5, #1
        add     r0, r0, r4
        b       .L4
.L3:
        .loc 1 19 0
        lsl     r7, r5, #1
        mov     ip, r7
        add     r4, r4, ip
        .loc 1 20 0
        add     r1, r1, r5
        .loc 1 21 0
        sub     r2, r2, r5
.L2:
        .loc 1 8 0
        cmp     r2, #0
        bgt     .L5
        b       .L6
.L4:
        .loc 1 30 0
        mov     r1, #0


The spilling is occurred at instruction A and reload at instruction B.

The spilled value is x. The source code computes next_runs and next_alpha
before while loop and preserve them through the loop body. But the generated
code preserve next_alpha, original runs and original x through the loop body
and compute next_runs after the loop. This caused an extra usage of register
and results in a register spilling.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[ramana at gcc dot gnu dot org]

------- Comment #2 from ramana at gcc dot gnu dot org  2009-06-12 12:54 -------
(In reply to comment #1)
> Created an attachment (id=17983)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17983&action=view) [edit]
> test case
> 

Your attachment didn't have #include <stdint.h> - Please try and supply
pre-processed input which is self contained . Adding a -I from a build
directory can be rather painful . Thanks - 

> The spilling is occurred around the first loop:
> 
>         push    {r4, r5, r6, r7, lr}
>         sub     sp, sp, #12
>         .loc 1 5 0
>         str     r2, [sp, #4]          // A
>         .loc 1 6 0
>         add     r6, r1, r2
>         mov     r4, r0
>         .loc 1 8 0
>         b       .L2
> .L5:
>         .loc 1 10 0
>         mov     r7, #0
>         ldrsh   r5, [r4, r7]
>         .loc 1 12 0
>         cmp     r2, r5
>         bge     .L3
>         .loc 1 14 0
>         ldrb    r7, [r1]
>         strb    r7, [r1, r2]
>         .loc 1 15 0
>         strh    r2, [r4]
>         .loc 1 16 0
>         lsl     r1, r2, #1
>         sub     r2, r5, r2
>         strh    r2, [r1, r4]
> .L6:
>         .loc 1 5 0
>         ldr     r5, [sp, #4]     //   B
>         lsl     r4, r5, #1
>         add     r0, r0, r4
>         b       .L4
> .L3:
>         .loc 1 19 0
>         lsl     r7, r5, #1
>         mov     ip, r7
>         add     r4, r4, ip
>         .loc 1 20 0
>         add     r1, r1, r5
>         .loc 1 21 0
>         sub     r2, r2, r5
> .L2:
>         .loc 1 8 0
>         cmp     r2, #0
>         bgt     .L5
>         b       .L6
> .L4:
>         .loc 1 30 0
>         mov     r1, #0
> 
> 
> The spilling is occurred at instruction A and reload at instruction B.
> 
> The spilled value is x. The source code computes next_runs and next_alpha
> before while loop and preserve them through the loop body. But the generated
> code preserve next_alpha, original runs and original x through the loop body
> and compute next_runs after the loop. This caused an extra usage of register
> and results in a register spilling.
> 

Could you say what you'd like the code to be because I don't see an option but
to spill one of the values here. ?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[carrot at google dot com]

------- Comment #3 from carrot at google dot com  2009-06-15 02:26 -------
Created an attachment (id=17998)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17998&action=view)
preprocessed test case

A possible code sequence without spilling is:

        push    {r4, r5, r6, r7, lr}
        add     r6, r1, r2
        mov     r4, r0
        lsl     r7, r2, 1     // New
        add     r0, r0, r7    // New
        .loc 1 8 0
        b       .L2
.L5:
        .loc 1 10 0
        mov     r7, #0
        ldrsh   r5, [r4, r7]
        .loc 1 12 0
        cmp     r2, r5
        bge     .L3
        .loc 1 14 0
        ldrb    r7, [r1]
        strb    r7, [r1, r2]
        .loc 1 15 0
        strh    r2, [r4]
        .loc 1 16 0
        lsl     r1, r2, #1
        sub     r2, r5, r2
        strh    r2, [r1, r4]
.L6:
        .loc 1 5 0
        b       .L4
.L3:
        .loc 1 19 0
        lsl     r7, r5, #1
        mov     ip, r7
        add     r4, r4, ip
        .loc 1 20 0
        add     r1, r1, r5
        .loc 1 21 0
        sub     r2, r2, r5
.L2:
        .loc 1 8 0
        cmp     r2, #0
        bgt     .L5
        b       .L6
.L4:
        .loc 1 30 0
        mov     r1, #0


-- 

carrot at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #17983|0                           |1
        is obsolete|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[carrot at google dot com]

------- Comment #4 from carrot at google dot com  2009-06-15 02:32 -------
In the source code, only two extra variables next_runs and next_alpha need to
be preserved through the while loop.

But in the gcc generated code, three variables are kept through the first loop.
They are next_alpha, original runs and original x. The expression (next_runs =
runs + x) is moved after the loop. This caused an extra var through the loop
and resulted in register spilling.

The expression move is occurred in tree-ssa-sink pass. Daniel Berlin has
confirmed it is a bug in this pass.

******** From Daniel **************
This looks like a bug, i think i know what causes it.
When I wrote this pass, i forgot to make this check:

 /* It doesn't make sense to move to a dominator that post-dominates
        frombb, because it means we've just moved it into a path that always
        executes if frombb executes, instead of reducing the number of
        executions .  */

     if (dominated_by_p (CDI_POST_DOMINATORS, frombb, commondom))

happen regardless of whether it is a single use statement or not.
So it will sink single use statements even if it's just moving them to
places that aren't executed less frequently.

Add that check (changing commondom to sinkbb) and it should stop moving it.
*********** End From Daniel ****************

I will send the patch later.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[ramana at gcc dot gnu dot org]

-- 

ramana at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-06-15 09:14:05
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[steven at gcc dot gnu dot org]

------- Comment #5 from steven at gcc dot gnu dot org  2009-06-22 16:32 -------
Did that patch go in already?


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |http://gcc.gnu.org/ml/gcc-
                   |                            |patches/2009-
                   |                            |06/msg01168.html


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[carrot at google dot com]

------- Comment #6 from carrot at google dot com  2009-06-30 07:42 -------
http://gcc.gnu.org/ml/gcc-cvs/2009-06/msg01067.html


-- 

carrot at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[steven at gcc dot gnu dot org]

-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416

[Bug target/40416] unnecessary register spill

[steven at gcc dot gnu dot org]

------- Comment #7 from steven at gcc dot gnu dot org  2009-06-30 08:21 -------
Please adjust the test case:

/* { dg-options "-O2 -Os -fdump-tree-sink-stats" } */

This makes no sense, -Os implies -O2.  So it should be:

/* { dg-options "-Os -fdump-tree-sink-stats" } */


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40416