[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.5                       |4.4.6

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.6                       |4.4.7

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jingyu at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

--- Comment #12 from Jing Yu <jingyu at google dot com> 2011-04-16 10:36:27 UTC ---
I am on leave from 02/01/2011 to 05/30/2011. I may not reply your
email during this period.

If you have Android toolchain questions/issues/requests, please
contact Doug (dougkwan@google.com) or my manager Bhaskar
(bjanakiraman@google.com).

Thanks,
Jing

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[lacombar at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

Arnaud Lacombe <lacombar at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lacombar at gmail dot com

--- Comment #13 from Arnaud Lacombe <lacombar at gmail dot com> 2011-05-13 19:26:40 UTC ---
It would seem that GCC 4.5.1 is still generating a call to func(), at -Os.
Considering the following reduced test-case:

extern int outcnt;
void test ()
{
 outcnt = 0;
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
 if (outcnt == 1) func ();
}

At -O2:

/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/cc1 -O2 -o - test.c
        .file   "test.c"
 test
Analyzing compilation unit
Performing interprocedural optimizations
 <*free_lang_data> <visibility> <early_local_cleanups> <whole-program> <cp>
<inline> <static-var> <pure-const>Assembling functions:
 test   .text
        .p2align 4,,15
.globl test
        .type   test, @function
test:
.LFB0:
        .cfi_startproc
        movl    $0, outcnt(%rip)
        ret
        .cfi_endproc
.LFE0:
        .size   test, .-test
        .ident  "GCC: (GNU) 4.5.1 20100924 (Red Hat 4.5.1-4)"
        .section        .note.GNU-stack,"",@progbits


Now, at -Os:

/usr/libexec/gcc/x86_64-redhat-linux/4.5.1/cc1 -Os -o - test.c
        .file   "test.c"
 test
Analyzing compilation unit
Performing interprocedural optimizations
 <*free_lang_data> <visibility> <early_local_cleanups> <whole-program> <cp>
<inline> <static-var> <pure-const>Assembling functions:
 test   .text
.globl test
        .type   test, @function
test:
.LFB0:
        .cfi_startproc
        movl    $0, outcnt(%rip)
        cmpl    $1, outcnt(%rip)
        jne     .L1
        xorl    %eax, %eax
        jmp     func
.L1:
        ret
        .cfi_endproc
.LFE0:
        .size   test, .-test
        .ident  "GCC: (GNU) 4.5.1 20100924 (Red Hat 4.5.1-4)"
        .section        .note.GNU-stack,"",@progbits

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jingyu at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

--- Comment #14 from Jing Yu <jingyu at google dot com> 2011-05-13 19:34:35 UTC ---
I am on leave from 02/01/2011 to 05/30/2011. I may not reply your
email during this period.

If you have Android toolchain questions/issues/requests, please
contact Doug (dougkwan@google.com) or my manager Bhaskar
(bjanakiraman@google.com).

Thanks,
Jing

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[lacombar at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

--- Comment #15 from Arnaud Lacombe <lacombar at gmail dot com> 2011-05-13 20:14:08 UTC ---
trunk from early May also fails with one more "if (outcnt == 1) func ();" line
at the end of the function:

% ./gcc/cc1 -Os -o - test.c

        .file   "test.c"
 test
Analyzing compilation unit
Performing interprocedural optimizations
 <*free_lang_data> <visibility> <early_local_cleanups> <whole-program>
<ipa-profile> <cp> <inline> <pure-const> <static-var>Assembling functions:
 test   .text
        .globl  test
        .type   test, @function
test:
.LFB0:
        .cfi_startproc
        movl    $0, outcnt(%rip)
        cmpl    $0, outcnt(%rip)
        je      .L1
        xorl    %eax, %eax
        jmp     func
.L1:
        ret
        .cfi_endproc
.LFE0:
        .size   test, .-test
        .ident  "GCC: (GNU) 4.7.0 20110416 (experimental)"
        .section        .note.GNU-stack,"",@progbits

Target and host are x86-64-linux. -O2 and above are fine.

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |

--- Comment #16 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-18 10:48:08 UTC ---
I don't see any calls to func() for the original testcase that survive at
-O2 or -Os for 4.5, 4.6 and current trunk.

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jingyu at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

--- Comment #17 from Jing Yu <jingyu at google dot com> 2011-05-18 11:06:35 UTC ---
I am on leave from 02/01/2011 to 05/30/2011. I may not reply your
email during this period.

If you have Android toolchain questions/issues/requests, please
contact Doug (dougkwan@google.com) or my manager Bhaskar
(bjanakiraman@google.com).

Thanks,
Jing

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[lacombar at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

--- Comment #18 from Arnaud Lacombe <lacombar at gmail dot com> 2011-05-18 15:17:48 UTC ---
Yes, the original test case is fine now, but not the updated snippet I posted.
Maybe should I open a new bug ?

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|4.4.7                       |4.5.0

--- Comment #19 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-03-13 13:04:09 UTC ---
Fixed in 4.5+, 4.4 is no longer supported.

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.4                       |4.4.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[steven at gcc dot gnu dot org]

------- Comment #11 from steven at gcc dot gnu dot org  2010-02-08 16:45 -------
Trunk today (r156595) optimizes this at -O1, -Os, and -O2 in the tree
optimizers. The .fre pass removes the first func call, then .dom1 removes the
next two. The .dom2 pass removes the remaining one.

If I add another, say, 10 lines of "if (outcnt == 1) func ();", then dom1
removes them all (even with -fno-tree-fre).

So something changed in (or for) DOM that allows it to clean up this mess
completely. I would like to know what... Perhaps the OP can try to bisect to
identify what was changed here (in the hope that it is something that can be
back-ported to GCC 4.4)?


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.3                       |4.4.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[steven at gcc dot gnu dot org]

------- Comment #10 from steven at gcc dot gnu dot org  2010-01-07 22:08 -------
An ARM maintainer can look at a solution for this (special splitters, maybe a
peephole2, perhaps a post-ce3 special DCE pass to clean up cond_exec silliness.
(See also bug 21803, for similar cond_exec idiocy on ia64.)


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rearnsha at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[steven at gcc dot gnu dot org]

-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |steven at gcc dot gnu dot
                   |                            |org
         AssignedTo|steven at gcc dot gnu dot   |unassigned at gcc dot gnu
                   |org                         |dot org
             Status|ASSIGNED                    |NEW


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[steven at gcc dot gnu dot org]

------- Comment #9 from steven at gcc dot gnu dot org  2010-01-07 21:36 -------
At -Os, in the .188r.postreload RTL dump:

(insn 46 12 17 2 t.c:8 (parallel [
            (set (reg/v:SI 2 r2 [orig:133 flags ] [133])
                (if_then_else:SI (ne (reg:SI 2 r2 [orig:138 bool_var ] [138])
                        (const_int 0 [0x0]))
                    (const_int 2 [0x2])
                    (const_int 0 [0x0])))
            (clobber (reg:CC 24 cc))
        ]) 291 {movcond} (nil))



And one pass later, in the .190r.split2 RTL dump:

(insn 49 12 50 2 t.c:8 (set (reg:CC 24 cc)
        (compare:CC (reg:SI 2 r2 [orig:138 bool_var ] [138])
            (const_int 0 [0x0]))) 220 {*arm_cmpsi_insn} (nil))

(insn 50 49 51 2 t.c:8 (cond_exec (ne (reg:CC 24 cc)
            (const_int 0 [0x0]))
        (set (reg/v:SI 2 r2 [orig:133 flags ] [133])
            (const_int 2 [0x2]))) 2367 {neon_vornv2di+77} (nil))

(insn 51 50 17 2 t.c:8 (cond_exec (eq (reg:CC 24 cc)
            (const_int 0 [0x0]))
        (set (reg/v:SI 2 r2 [orig:133 flags ] [133])
            (const_int 0 [0x0]))) 2367 {neon_vornv2di+77} (nil))


There's your redundant mov, from an insn splitter.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[steven at gcc dot gnu dot org]

------- Comment #8 from steven at gcc dot gnu dot org  2010-01-07 21:29 -------
The diff between -O2 and -Os starts in combine:

diff -ur O2/t.c.175r.combine Os/t.c.175r.combine
--- O2/t.c.175r.combine 2010-01-07 22:24:04.000000000 +0100
+++ Os/t.c.175r.combine 2010-01-07 22:23:14.000000000 +0100
@@ -4,18 +4,20 @@
 starting the processing of deferred insns
 ending the processing of deferred insns
 df_analyze called
-insn_cost 7: 12
+insn_cost 7: 8
 insn_cost 8: 4
 insn_cost 9: 4
-insn_cost 10: 12
-insn_cost 11: 12
+insn_cost 10: 8
+insn_cost 11: 4
 insn_cost 12: 4
-insn_cost 46: 4
-insn_cost 17: 12
+insn_cost 46: 16
+insn_cost 17: 8
 insn_cost 19: 4
-rejecting combination of insns 12 and 46
-original costs 4 + 4 = 8
-replacement cost 12
+deferring deletion of insn with uid = 12.
+modifying insn i3    46 {r133:SI={(r138:SI!=0x0)?0x2:0x0};clobber cc:CC;}
+      REG_UNUSED: cc:CC
+      REG_DEAD: r138:SI
+deferring rescan insn with uid = 46.
 (note# 0 # 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

 (note# # # 2 NOTE_INSN_FUNCTION_BEG)
@@ -40,16 +42,17 @@
         (expr_list:REG_EQUAL (mem/c/i:SI (symbol_ref:SI ("bool_var") [flags
0xc0]  <var_decl # bool_var>) [2 bool_var+0 S4 A32])
             (nil))))

-(insn# # # 2 t.c:13 (set (reg:CC 24 cc)
-        (compare:CC (reg:SI 138 [ bool_var ])
-            (const_int 0 [0x0])))# {*arm_cmpsi_insn} (nil))
-
-(insn# # # 2 t.c:8 (set (reg/v:SI 133 [ flags ])
-        (if_then_else:SI (eq (reg:CC 24 cc)
-                (const_int 0 [0x0]))
-            (reg:SI 138 [ bool_var ])
-            (const_int 2 [0x2])))# {*movsicc_insn} (expr_list:REG_DEAD (reg:SI
138 [ bool_var ])
-        (expr_list:REG_DEAD (reg:CC 24 cc)
+(note# # # 2 NOTE_INSN_DELETED)
+
+(insn# # # 2 t.c:8 (parallel [
+            (set (reg/v:SI 133 [ flags ])
+                (if_then_else:SI (ne (reg:SI 138 [ bool_var ])
+                        (const_int 0 [0x0]))
+                    (const_int 2 [0x2])
+                    (const_int 0 [0x0])))
+            (clobber (reg:CC 24 cc))
+        ])# {movcond} (expr_list:REG_UNUSED (reg:CC 24 cc)
+        (expr_list:REG_DEAD (reg:SI 138 [ bool_var ])
             (nil))))

 (insn# # # 2 t.c:14 (set (reg/f:SI 139)
@@ -60,7 +63,10 @@
         (expr_list:REG_DEAD (reg/v:SI 133 [ flags ])
             (nil))))
 starting the processing of deferred insns
+deleting insn with uid = 12.
+rescanning insn with uid = 46.
+deleting insn with uid = 46.
 ending the processing of deferred insns

-;; Combiner totals: 12 attempts, 12 substitutions (2 requiring new space),
-;; 0 successes.
+;; Combiner totals: 13 attempts, 13 substitutions (2 requiring new space),
+;; 1 successes.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[steven at gcc dot gnu dot org]

------- Comment #7 from steven at gcc dot gnu dot org  2010-01-07 09:49 -------
*sigh* With all the Google power, why can't anyone there try to debug this?

I'll have a look.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |steven at gcc dot gnu dot
                   |dot org                     |org
             Status|REOPENED                    |ASSIGNED
   Last reconfirmed|2009-12-29 10:35:21         |2010-01-07 09:49:13
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[sliao at google dot com]

------- Comment #6 from sliao at google dot com  2010-01-07 09:18 -------
For the trunk snapshot of 20100102, GCC 4.5.0 indeed removes most of the
redundancy. However, -O1 and -Os still produce an extra instruction, while -O2
doesn't. Do we care about an extra instruction below? Thanks.

The instruction is marked as "redundant" below.

GCC trunk's -Os:
Disassembly of section .text:
00000000 <test>:
   0:   e59f3024        ldr     r3, [pc, #36]   ; 2c <test+0x2c>
   4:   e3a02000        mov     r2, #0
   8:   e5832000        str     r2, [r3]
   c:   e59f301c        ldr     r3, [pc, #28]   ; 30 <test+0x30>
  10:   e5932000        ldr     r2, [r3]
  14:   e59f3018        ldr     r3, [pc, #24]   ; 34 <test+0x34>
  18:   e3520000        cmp     r2, #0
  1c:   13a02002        movne   r2, #2 
  20:   03a02000        moveq   r2, #0 ;redundant
  24:   e5c32000        strb    r2, [r3]
  28:   e12fff1e        bx      lr
        ...

GCC trunk's -O1:
Disassembly of section .text:
00000000 <test>:
   0:   e3a02000        mov     r2, #0
   4:   e59f3020        ldr     r3, [pc, #32]   ; 2c <test+0x2c>
   8:   e5832000        str     r2, [r3]
   c:   e59f301c        ldr     r3, [pc, #28]   ; 30 <test+0x30>
  10:   e5932000        ldr     r2, [r3]
  14:   e3520000        cmp     r2, #0
  18:   13a02002        movne   r2, #2
  1c:   03a02000        moveq   r2, #0  ;redundant
  20:   e59f300c        ldr     r3, [pc, #12]   ; 34 <test+0x34>
  24:   e5c32000        strb    r2, [r3]
  28:   e12fff1e        bx      lr
        ...

Below, GCC trunk's -O2 doesn't produce the redundant instruction.
-O2:
Disassembly of section .text:
00000000 <test>:
   0:   e59f3020        ldr     r3, [pc, #32]   ; 28 <test+0x28>
   4:   e59f2020        ldr     r2, [pc, #32]   ; 2c <test+0x2c>
   8:   e5933000        ldr     r3, [r3]
   c:   e3a01000        mov     r1, #0
  10:   e3530000        cmp     r3, #0
  14:   e5821000        str     r1, [r2]
  18:   e59f2010        ldr     r2, [pc, #16]   ; 30 <test+0x30>
  1c:   13a03002        movne   r3, #2
  20:   e5c23000        strb    r3, [r2]
  24:   e12fff1e        bx      lr
        ...


-- 

sliao at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jingyu at google dot com,
                   |                            |dougkwan at google dot com,
                   |                            |carrot at google dot com
             Status|RESOLVED                    |REOPENED
         Resolution|WONTFIX                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2009-12-31 15:36 -------
Btw, in 4.4 there is one DOM pass removed for compile-time.

We won't fix this for 4.4, the particular testcase is fixed in 4.5.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to fail|                            |4.4.3
      Known to work|4.2.1                       |4.2.1 4.5.0
         Resolution|                            |WONTFIX


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2009-12-30 23:34 -------
The issue is that we do not do predicated value-numbering so we need multiple
invocations of it.  Which all of FRE, DOM and PRE do - but PRE is not run at
-Os in 4.4.

And yes, the optimization done by 4.5 is correct.

You could say "fixed" in 4.5 - or wait for someone to implement predicated
value-numbering to also fix a testcase with one more if (outcnt == 1) func ();


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matz at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494

[Bug tree-optimization/42494] [4.4 Regression] Missed dead-code-elimination

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.4.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42494