[Bug target/42258] New: redundant register move around mul instruction

[Bug target/42258] New: redundant register move around mul instruction

[carrot at google dot com]

Compile following function with options -march=armv5te -mthumb -Os

int mulx(int x)
{
  return x*42;
}

Gcc generates:

mulx:
        mov     r2, #42
        mov     r3, r2    //A
        mul     r3, r0
        @ sp needed for prologue
        mov     r0, r3    //B
        bx      lr

There are two redundant mov instructions marked as A and B. The ideal code
should be:

    mov r2, #42
    mul r0, r2
    bx  lr

The root causes of the two movs are different.

A. 
In arm.md there is an insn pattern which can support 3-register multiplication.

(define_insn "*thumb_mulsi3"
  [(set (match_operand:SI          0 "register_operand" "=&l,&l,&l")
        (mult:SI (match_operand:SI 1 "register_operand" "%l,*h,0")
                 (match_operand:SI 2 "register_operand" "l,l,l")))]
  "TARGET_THUMB1 && !arm_arch6"
  "*
  if (which_alternative < 2)
    return \"mov\\t%0, %1\;mul\\t%0, %2\";
  else
    return \"mul\\t%0, %2\";
  "
  [(set_attr "length" "4,4,2")
   (set_attr "insn" "mul")]
)

So after expand we got the following rtx insn:

(insn 7 6 8 3 testD.i:2 (set (reg:SI 136)
        (mult:SI (reg:SI 137)
            (reg/v:SI 135 [ x ]))) -1 (nil))

For register allocator there is no difference between (set r3 mult(r0, r2)) 
and   (set r2 mult(r0, r2))   if the original r2 is dead after this
instruction. Unfortunately the first form is chosen, so an extra mov is
generated.

B.
After rtl expansion I got:

(insn 2 4 3 2 testD.i:2 (set (reg/v:SI 135 [ x ])
        (reg:SI 0 r0 [ x ])) -1 (nil))

(note 3 2 5 2 NOTE_INSN_FUNCTION_BEG)

(note 5 3 6 3 [bb 3] NOTE_INSN_BASIC_BLOCK)

(insn 6 5 7 3 testD.i:2 (set (reg:SI 137)
        (const_int 42 [0x2a])) -1 (nil))

(insn 7 6 8 3 testD.i:2 (set (reg:SI 136)
        (mult:SI (reg:SI 137)
            (reg/v:SI 135 [ x ]))) -1 (nil))

(insn 8 7 9 3 testD.i:2 (set (reg:SI 134 [ <retval> ])
        (reg:SI 136)) -1 (nil))

(jump_insn 9 8 10 3 testD.i:2 (set (pc)
        (label_ref 11)) -1 (nil)
 -> 11)

(barrier 10 9 16)

(note 16 10 13 4 [bb 4] NOTE_INSN_BASIC_BLOCK)

(insn 13 16 14 4 testD.i:4 (clobber (reg/i:SI 0 r0)) -1 (nil))

(insn 14 13 11 4 testD.i:4 (clobber (reg:SI 134 [ <retval> ])) -1 (nil))

(code_label 11 14 17 5 1 "" [1 uses])

(note 17 11 12 5 [bb 5] NOTE_INSN_BASIC_BLOCK)

(insn 12 17 15 5 testD.i:4 (set (reg/i:SI 0 r0)
        (reg:SI 134 [ <retval> ])) -1 (nil))

(insn 15 12 0 5 testD.i:4 (use (reg/i:SI 0 r0)) -1 (nil))

The extra mov comes from insn 12 and no later pass can eliminate it. Should we
propagate expression into return register?


-- 
           Summary: redundant register move around mul instruction
           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=42258

[Bug regression/42258] redundant register move around mul instruction

[carrot at google dot com]

------- Comment #1 from carrot at google dot com  2009-12-09 02:27 -------
Gcc 4.4 doesn't have this problem. It is a new regression caused by patch
152533.


-- 

carrot at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vmakarov at redhat dot com
          Component|target                      |regression


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug regression/42258] redundant register move around mul instruction

[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         |2009-12-09 16:12:19
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|regression                  |rtl-optimization
           Keywords|                            |missed-optimization, ra
            Summary|[4.5 only] redundant        |[4.5 Regression] redundant
                   |register move around mul    |register move around mul
                   |instruction                 |instruction
   Target Milestone|---                         |4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2009-12-31 09:32 -------
http://gcc.gnu.org/viewcvs?view=revision&revision=152533


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |22072


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2009-12-31 11:11 -------
But the patch was backported.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[bernds at codesourcery dot com]

------- Comment #4 from bernds at codesourcery dot com  2010-03-17 11:05 -------
It's not immediately obvious to me why the ARM mulsi3 patterns are written the
way they are - what are the earlyclobber tricks supposed to be good for? 
Richard E., any clues?


-- 

bernds at codesourcery dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rearnsha at arm dot com,
                   |                            |bernds at codesourcery dot
                   |                            |com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[bernds at codesourcery dot com]

------- Comment #5 from bernds at codesourcery dot com  2010-03-17 11:44 -------
Created an attachment (id=20123)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=20123&action=view)
A patch to fix it.

Okay, so the pattern is written strangely because it's a two-operand mul where
the input and output may not be the same register.
It may be better to write this using an earlyclobbered in-out operand, but you
can't show anymore that the operation is commutative.

It can be fixed with a simple peephole optimization, at least for this
testcase.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2010-03-17 11:59 -------
Perhaps add a comment why the peephole is needed? That information tend to get
lost rather quickly.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[bernds at gcc dot gnu dot org]

------- Comment #7 from bernds at gcc dot gnu dot org  2010-03-19 18:19 -------
Subject: Bug 42258

Author: bernds
Date: Fri Mar 19 18:18:54 2010
New Revision: 157581

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=157581
Log:
gcc/
        PR rtl-optimization/42258
        * ira-lives.c (check_and_make_def_conflict): Ignore conflict for a
        use that may match DEF.

testsuite/
        PR rtl-optimization/42258
        * gcc.target/arm/thumb1-mul-moves.c: New test.



Added:
    trunk/gcc/testsuite/gcc.target/arm/thumb1-mul-moves.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/ira-lives.c
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258

[Bug rtl-optimization/42258] [4.5 Regression] redundant register move around mul instruction

[steven at gcc dot gnu dot org]

------- Comment #8 from steven at gcc dot gnu dot org  2010-03-20 12:58 -------
Shouldeth be fixedeth by aforementionedeth patcheth (comment #7). Yay!


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |http://gcc.gnu.org/ml/gcc-
                   |                            |patches/2010-
                   |                            |03/msg00905.html
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42258