[Bug c/40987] New: Wrong optimization with if-conversion

[Bug c/40987] New: Wrong optimization with if-conversion

[thomas at coware dot com]

The following test fails when I disable all optimizations but if-conversion:

#include <stdio.h>

long long func(long arg) {
   long long val = 0;
   if (arg < 0) {
      val = 0xffffffff80000000ull;
   }
   return val;
}

int main() {
   long long result = func(-1);
   if (result != 0xffffffff80000000ull) {
      printf("failed.\n");
      return 1;
   } 
   printf("passed.\n");
   return 0;
}

The return value of func is 0x80000000ull where it should be
0xffffffff80000000ull. It works correctly when I don't do any optimization or
with more optimization (-O1, -O2, -O3).

The compile command line is the following:
gcc test.c -O1 -fno-defer-pop -fno-delayed-branch -fno-guess-branch-probability
-fno-cprop-registers -fno-loop-optimize -fno-if-conversion2 -fno-tree-ccp
-fno-tree-dce -fno-tree-dominator-opts -fno-tree-dse -fno-tree-ter 
-fno-tree-lrs -fno-tree-sra -fno-tree-copyrename -fno-tree-fre -fno-tree-ch
-fno-unit-at-a-time -fno-merge-constants -fno-omit-frame-pointer -o test

there are no compiler warnings, even with -Wall

I use gcc 4.3.4, configure options:
 ../gcc-4.3.4/configure --disable-shared --disable-nls --enable-languages=c++
--with-gnu-as --with-gnu-ld --enable-__cxa_atexit


-- 
           Summary: Wrong optimization with if-conversion
           Product: gcc
           Version: 4.3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: thomas at coware dot com
 GCC build triplet: i686
  GCC host triplet: i686
GCC target triplet: i686


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2009-08-06 14:57 -------
"Doctor it hurts when I do this!" ...

well, I suggest you don't use this strange combination of options.  In
particular
disabling ccp is known to cause interesting side-effects sometimes.

Keeping open in case someone wants to track down what happens here.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[thomas at coware dot com]

------- Comment #2 from thomas at coware dot com  2009-08-06 15:26 -------
I can also build it like this and it fails:
gcc test.c -O1 -fno-tree-dce -o test


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[mikpe at it dot uu dot se]

------- Comment #3 from mikpe at it dot uu dot se  2009-08-06 15:58 -------
Reproduced on i686-linux with the following compiler versions and minimal sets
of options:

gcc-4.3-20090802: -O1 -fno-tree-dce
gcc-4.4-20090804: -O1 -fno-tree-ccp -fno-tree-dominator-opts
gcc-4.5-20090730: -O1

Note that 4.5 needed no -fno-... options to create the failure.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[thomas at coware dot com]

------- Comment #4 from thomas at coware dot com  2009-08-06 19:25 -------
By the way, I see the same failure also with gcc 4.1.2 and 4.2 without any fno-
options.

I thought it would be a good idea to hunt the problem down to a single
optimization mechanism, which seems to be if-conversion. I guess on 4.3 and 4.4
some of the tree optimizations change the code in a way that the problem is not
visible - but it is most likely there anyway and might be triggered by other
test cases.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[steven at gcc dot gnu dot org]

------- Comment #5 from steven at gcc dot gnu dot org  2009-08-06 22:12 -------
Instead of guessing -- why not inspect the dumps you get with -fdump-tree-all
-fdump-rtl-all and see if someone can figure out which pass is the first to
screw up the test case?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2009-08-06 22:13 -------
If it turns out to be the RTL if-conversion pass, assign this bug to me please.


-- 

steven at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug c/40987] Wrong optimization with if-conversion

[thomas at coware dot com]

------- Comment #7 from thomas at coware dot com  2009-08-07 07:37 -------
This is how function func looks after the if conversion (test.c.144r.ce1):

(note 4 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

(insn 2 4 3 2 test.c:3 (set (reg/v:SI 63 [ arg ])
        (mem/c/i:SI (reg/f:SI 16 argp) [0 arg+0 S4 A32])) 41 {*movsi_1}
(expr_list:REG_EQUIV (mem/c/i:SI (reg/f:SI 16 argp) [0 arg+0 S4 A32])
        (nil)))

(note 3 2 7 2 NOTE_INSN_FUNCTION_BEG)

(insn 7 3 34 2 test.c:5 (set (reg:CCGOC 17 flags)
        (compare:CCGOC (reg/v:SI 63 [ arg ])
            (const_int 0 [0x0]))) 0 {*cmpsi_ccno_1} (nil))

(insn 34 7 35 2 test.c:5 (parallel [
            (set (reg:DI 65)
                (sign_extend:DI (reg/v:SI 63 [ arg ])))
            (clobber (reg:CC 17 flags))
            (clobber (scratch:SI))
        ]) 87 {*extendsidi2_1} (nil))

(insn 35 34 36 2 test.c:5 (parallel [
            (set (reg/v:DI 60 [ val ])
                (lshiftrt:DI (reg:DI 65)
                    (const_int 63 [0x3f])))
            (clobber (reg:CC 17 flags))
        ]) 356 {*lshrdi3_1} (nil))

(insn 36 35 24 2 test.c:5 (parallel [
            (set (reg/v:DI 60 [ val ])
                (ashift:DI (reg/v:DI 60 [ val ])
                    (const_int 31 [0x1f])))
            (clobber (reg:CC 17 flags))
        ]) 319 {*ashldi3_1} (nil))

(insn 24 36 30 2 test.c:9 (set (reg/i:DI 0 ax)
        (reg/v:DI 60 [ val ])) 63 {*movdi_2} (nil))

(insn 30 24 0 2 test.c:9 (use (reg/i:DI 0 ax)) -1 (nil))

This already looks wrong to me, but I am not completely sure. Could somebody
have a look who understands this RTL code better than me?

The last version before the if conversion looks like this (test.c.141r.cse1):
(note 4 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)

(insn 2 4 3 2 test.c:3 (set (reg/v:SI 63 [ arg ])
        (mem/c/i:SI (reg/f:SI 16 argp) [0 arg+0 S4 A32])) 41 {*movsi_1}
(expr_list:REG_EQUIV (mem/c/i:SI (reg/f:SI 16 argp) [0 arg+0 S4 A32])
        (nil)))

(note 3 2 7 2 NOTE_INSN_FUNCTION_BEG)

(insn 7 3 8 2 test.c:5 (set (reg:CCGOC 17 flags)
        (compare:CCGOC (reg/v:SI 63 [ arg ])
            (const_int 0 [0x0]))) 0 {*cmpsi_ccno_1} (nil))

(jump_insn 8 7 9 2 test.c:5 (set (pc)
        (if_then_else (lt (reg:CCGOC 17 flags)
                (const_int 0 [0x0]))
            (label_ref 13)
            (pc))) 401 {*jcc_1} (nil))

(note 9 8 10 3 [bb 3] NOTE_INSN_BASIC_BLOCK)

(insn 10 9 13 3 test.c:5 (set (reg/v:DI 60 [ val ])
        (const_int 0 [0x0])) 63 {*movdi_2} (nil))

(code_label 13 10 14 4 2 "" [1 uses])

(note 14 13 16 4 [bb 4] NOTE_INSN_BASIC_BLOCK)

(insn 16 14 17 4 test.c:6 (set (reg/v:DI 60 [ val ])
        (const_int -2147483648 [0x80000000])) 63 {*movdi_2} (nil))

(code_label 17 16 18 5 3 "" [0 uses])

(note 18 17 20 5 [bb 5] NOTE_INSN_BASIC_BLOCK)

(insn 20 18 24 5 test.c:8 (set (reg:DI 62 [ <result> ])
        (reg/v:DI 60 [ val ])) 63 {*movdi_2} (nil))

(insn 24 20 30 5 test.c:9 (set (reg/i:DI 0 ax)
        (reg/v:DI 60 [ val ])) 63 {*movdi_2} (nil))

(insn 30 24 0 5 test.c:9 (use (reg/i:DI 0 ax)) -1 (nil))


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug rtl-optimization/40987] Wrong optimization with if-conversion

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2009-08-07 12:10 -------
Seems to be a HWI32 issue as I cannot reproduce the fail on x86_64 with -m32.
Which would indeed hint at a RTL if-conversion problem (I guess it uses
scc in a wrong way).


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
          Component|c                           |rtl-optimization
           Keywords|                            |wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug rtl-optimization/40987] Wrong optimization with if-conversion

[mikpe at it dot uu dot se]

------- Comment #9 from mikpe at it dot uu dot se  2009-09-19 16:57 -------
Seems like an if-conversion bug, in particular noce_try_store_flag_constants()
appears to break on HWI32 platforms when long long literals are involved.

In this test case, noce_t_s_f_c() is invoked with an IF where a (the false
value) and b (the true value) are both DImode CONST_INTs. a is zero. b is
stored as 0x80000000, which is truncated but sign-extends to the correct value.

noce_t_s_f_c() then computes a subtraction and some log2() on these truncated
values, and selects the second code generation template "x = (test != 0) <<
3;".
The code becomes (from the ce1 dump file):

(insn 27 8 28 2 pr40987.c:4 (parallel [
            (set (reg:DI 62)
                (sign_extend:DI (reg/v:SI 60 [ arg ])))
            (clobber (reg:CC 17 flags))
            (clobber (scratch:SI))
        ]) 90 {*extendsidi2_1} (nil))

(insn 28 27 29 2 pr40987.c:4 (parallel [
            (set (reg/v:DI 58 [ val ])
                (lshiftrt:DI (reg:DI 62)
                    (const_int 63 [0x3f])))
            (clobber (reg:CC 17 flags))
        ]) 394 {*lshrdi3_1} (nil))

# since this is a logical downshift, it produces 0 or 1 from the sign bit

(insn 29 28 17 2 pr40987.c:4 (parallel [
            (set (reg/v:DI 58 [ val ])
                (ashift:DI (reg/v:DI 58 [ val ])
                    (const_int 31 [0x1f])))
            (clobber (reg:CC 17 flags))
        ]) 356 {*ashldi3_1} (nil))

# which is shifted up to produce 0 or 0x80000000, the high 32 bits are lost

All gcc-4.x releases have this bug. It does not trigger in gcc-3.4.6. There it
seems that the true/false values are swapped compared to 4.x, and a test near
the top of noce_t_s_f_c() causes it to bail out and not perform the conversion.
(The test looks the same in 4.x, but the IF-operands are swapped so it does not
bail.)

I don't know if this code can be fixed to handle longer-than-HWI literals. The
simplest solution might be to just detect the situation and bail out.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug rtl-optimization/40987] Wrong optimization with if-conversion

[mikpe at it dot uu dot se]

------- Comment #10 from mikpe at it dot uu dot se  2009-09-24 14:38 -------
Patch posted for review:
http://gcc.gnu.org/ml/gcc-patches/2009-09/msg01655.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987

[Bug rtl-optimization/40987] Wrong optimization with if-conversion

[steven at gcc dot gnu dot org]

-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |http://gcc.gnu.org/ml/gcc-
                   |                            |patches/2009-
                   |                            |09/msg01655.html
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |patch
   Last reconfirmed|0000-00-00 00:00:00         |2009-09-26 10:18:29
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40987