[Bug rtl-optimization/45813] New: alias analysis problem ?

[Bug rtl-optimization/45813] New: alias analysis problem ?

[darrenrjenkins at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

           Summary: alias analysis problem ?
           Product: gcc
           Version: 4.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: darrenrjenkins@gmail.com


I think I am having some wrong code generated due to a compiler bug.
I am compiling for ARM/Thumb.

USB_INT16U  ReadLE16U ( volatile USB_INT08U  *pmem )
{
    USB_INT16U  val;
    USB_INT08U *bytes = (USB_INT08U *)&val;

    bytes[0] = pmem[0];
    bytes[1] = pmem[1];

    return (val);
}

gives me

    B580        push {r7, lr}
    B081        sub sp, #4
    AF00        add r7, sp, #0
    1CBB        adds r3, r7, #2
    7802        ldrb r2, [r0, #0]
    701A        strb r2, [r3, #0]
    7842        ldrb r2, [r0, #1]
    8818        ldrh r0, [r3, #0]
    46BD        mov sp, r7
    B001        add sp, #4
    BC80        pop {r7}
    BC02        pop {r1}
    4708        bx r1

Which reads the second byte into r2 but does not store it on the stack, so the
second byte of val is returned uninitialized.
It looks like the alias analysis knows that bytes points to val but doesn't
know that bytes+1 points to a part of val also, so optimizes it away.

Yell out if you need any more information

[Bug rtl-optimization/45813] alias analysis problem ?

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2010.09.28 09:17:33
               date|                            |
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-09-28 09:17:33 UTC ---
I can't reproduce this on x86_64-linux with GCC 4.4.4 and -O2 (what options
did you use?).  A cross to arm I still have around for GCC 4.5.0 also doesn't
show the problem.

I used

unsigned short ReadLE16U ( volatile unsigned char *pmem)
{
  unsigned short val;
  unsigned char *bytes = (unsigned char *)&val;
  bytes[0] = pmem[0];
  bytes[1] = pmem[1];
  return val;
}

if that is not enough to reproduce the problem please provide
complete preprocessed source that can be compiled.

[Bug rtl-optimization/45813] alias analysis problem ?

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

Mikael Pettersson <mikpe at it dot uu.se> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikpe at it dot uu.se

--- Comment #2 from Mikael Pettersson <mikpe at it dot uu.se> 2010-09-28 11:24:18 UTC ---
I can't reproduce this with a freshly-built vanilla 4.4.4 for
arm-linux-gnueabi.
Please tell us (a) exactly how your compiler was configured, and (b) exactly
how you invoked it on the test case.

[Bug rtl-optimization/45813] alias analysis problem ?

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #4 from Mikael Pettersson <mikpe at it dot uu.se> 2010-09-28 23:21:27 UTC ---
Please show us the output of "gcc -v", we want to see how the compiler was
configured (wrt certain defaults etc).  Also, when you invoke the compiler we
want to see the exact options YOU supply to it; right now it's difficult to
separate your options from what the compiler driver passes to the backend.

[Bug rtl-optimization/45813] alias analysis problem ?

[darrenrjenkins at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #3 from Darren Jenkins <darrenrjenkins at gmail dot com> 2010-09-28 23:07:08 UTC ---
unsigned short ReadLE16U( volatile unsigned char * ptr )
{
    unsigned short value;
    unsigned char * bytes = (unsigned char *)&value;

    bytes[0] = ptr[0];
    bytes[1] = ptr[1];

    return value;
}

Gives me the same erroneous results.

The compiler I am using is part of Rowley "CrossWorks for ARM" 2.0.7 which
claims to be an unmodified GCC 4.4.4
http://www.rowley.co.uk/crossworks/gpl_sources.htm

cc1 --version
GNU C (GCC) version 4.4.4 (arm-unknown-elf)
        compiled by GNU C version 3.4.4 (mingw special), GMP version 4.3.2
 version 2.4.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

What gets passed to the compiler seems to be :

"C:/Program Files (x86)/Rowley Associates Limited/CrossWorks for ARM
2.0/gcc/bin/cc1" -fmessage-length=0 -mcpu=arm7tdmi-s -mthumb -mthumb-interwork
-mlittle-endian -mfpu=vfp -mfloat-abi=soft -nostdinc "-isystemC:/Program Files
(x86)/Rowley Associates Limited/CrossWorks for ARM 2.0/include"
"-isystemC:/Users/DarrenJenkins/AppData/Local/Rowley Associates
Limited/CrossWorks for ARM/packages/include" -I. -I../.. -I../../include
-I../../system -I../../LwIP -I../../LwIP/include -I../../LwIP/include/lwip
-I../../LwIP/include/ipv4 -I../../NXP_Lib -I../../fat_file_system -I../../usb
-I../../usb/UsbHost/Include -I../../Macro -D__ARM_ARCH_4T__ -D__CROSSWORKS_ARM
-D__CROSSWORKS_MAJOR_VERSION=2 -D__CROSSWORKS_MINOR_VERSION=0
-D__CROSSWORKS_REVISION=7 -D__TARGET_PROCESSOR=LPC2468 -DNESTED_INTERRUPTS
-DSRAM_EXCEPTIONS -D__THUMB -D__FLASH_BUILD
-DOSCILLATOR_CLOCK_FREQUENCY=12000000 -DDEBUG -MD "THUMB Flash
Debug/usbhost_lpc2468.d" -MQ "THUMB Flash Debug/usbhost_lpc2468.o" -quiet -Wall
-fno-omit-frame-pointer -fno-schedule-insns2 -gdwarf-2 -Os -fno-dwarf2-cfi-asm
-fno-builtin -ffunction-sections -fdata-sections
C:/Users/DarrenJenkins/Documents/NXP_darren/src/projects/Darren/../../usb/UsbHost/Host/usbhost_lpc2468.c
-o "C:/Users/DarrenJenkins/Documents/NXP_darren/src/projects/Darren/THUMB Flash
Debug/usbhost_lpc2468.asm"

and 

"C:/Program Files (x86)/Rowley Associates Limited/CrossWorks for ARM
2.0/gcc/bin/as" --traditional-format -mcpu=arm7tdmi-s -mthumb -mthumb-interwork
-EL -mfpu=vfp -mfloat-abi=soft
"C:/Users/DarrenJenkins/Documents/NXP_darren/src/projects/Darren/THUMB Flash
Debug/usbhost_lpc2468.asm" -o
"C:/Users/DarrenJenkins/Documents/NXP_darren/src/projects/Darren/THUMB Flash
Debug/usbhost_lpc2468.o"


Yell out if you would like to know anything else.

[Bug rtl-optimization/45813] alias analysis problem ?

[darrenrjenkins at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #5 from Darren Jenkins <darrenrjenkins at gmail dot com> 2010-09-29 03:33:27 UTC ---
> Please show us the output of "gcc -v", we want to see how the compiler was
> configured (wrt certain defaults etc).  Also, when you invoke the compiler we
> want to see the exact options YOU supply to it; right now it's difficult to
> separate your options from what the compiler driver passes to the backend.

This is a little difficult as the compiler is part of a whole
"development/build environment" thing, and I can't find an executable called
"gcc" in it.

I will investigate how it is put together and see what I can find out.

[Bug rtl-optimization/45813] alias analysis problem ?

[darrenrjenkins at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #6 from Darren Jenkins <darrenrjenkins at gmail dot com> 2010-09-29 03:55:39 UTC ---
Also if I don't use -fno-omit-frame-pointer the code seems to be generated
correctly

[Bug rtl-optimization/45813] alias analysis problem ?

[darrenrjenkins at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #7 from Darren Jenkins <darrenrjenkins at gmail dot com> 2010-09-29 07:46:54 UTC ---
OK CrossWorks don't use/ship the GCC compiler driver.
So I guess all the usefull information I can give is from the c compiler:

cc1 --version
GNU C (GCC) version 4.4.4 (arm-unknown-elf)
        compiled by GNU C version 3.4.4 (mingw special), GMP version 4.3.2
 version 2.4.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

[Bug rtl-optimization/45813] alias analysis problem ?

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |UNCONFIRMED
     Ever Confirmed|1                           |0

--- Comment #8 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-09-29 09:14:46 UTC ---
Should be enough information for arm people to confirm.  I still suggest
to report the problem to Rowley CrossWorks instead.

[Bug rtl-optimization/45813] alias analysis problem ?

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #9 from Mikael Pettersson <mikpe at it dot uu.se> 2010-09-29 19:41:08 UTC ---
I can reproduce it with crosses to arm-elf and arm-linux-gnueabi.  The
combination of -mthumb (1 not 2) and -fno-omit-frame-pointer is the trigger.
GCC 4.3.5 works, 4.4 and 4.5 generate broken code, 4.6 works.

gcc-4.3.5 -march=armv5te -mthumb -fno-omit-frame-pointer -Os generates:

ReadLE16U:
        push    {r7, lr}
        ldrb    r3, [r0]
        sub     sp, sp, #8
        add     r7, sp, #0
        add     r2, r7, #6
        strb    r3, [r2]
        ldrb    r3, [r0, #1]
        mov     sp, r7
        add     sp, sp, #8
        strb    r3, [r2, #1]
        ldrh    r0, [r2]
        @ sp needed for prologue
        pop     {r7, pc}

The second byte is correctly stored before the short is read and returned.

gcc-4.4-20100928 and gcc-4.5-20100923 generate:

ReadLE16U:
        push    {r7, lr}
        ldrb    r2, [r0]
        sub     sp, sp, #8
        add     r7, sp, #0
        add     r3, r7, #6
        mov     sp, r7
        strb    r2, [r3]
        add     sp, sp, #8
        ldrb    r2, [r0, #1]
        @ sp needed for prologue
        ldrh    r0, [r3]
        pop     {r7, pc}

The second byte is read but not written before the short is read.

gcc-4.6-20100925 generates:

ReadLE16U:
        push    {r0, r1, r7, lr}
        ldrb    r3, [r0]
        add     r7, sp, #0
        mov     sp, r7
        strb    r3, [r7, #6]
        ldrb    r3, [r0, #1]
        @ sp needed for prologue
        strb    r3, [r7, #7]
        ldrh    r0, [r7, #6]
        pop     {r1, r2, r7, pc}

This is correct.

[Bug rtl-optimization/45813] alias analysis problem ?

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #10 from Mikael Pettersson <mikpe at it dot uu.se> 2010-09-29 20:59:17 UTC ---
This was fixed for 4.6 by r160260:
http://gcc.gnu.org/viewcvs?view=revision&revision=160260

Applying that to 4.5 changes 4.5's output to:

ReadLE16U:
        push    {r7, lr}
        ldrb    r3, [r0]
        sub     sp, sp, #8
        add     r7, sp, #0
        strb    r3, [r7, #6]
        ldrb    r3, [r0, #1]
        mov     sp, r7
        add     sp, sp, #8
        strb    r3, [r7, #7]
        ldrh    r0, [r7, #6]
        @ sp needed for prologue
        pop     {r7, pc}

which is much better.  (It still runs into PR38644, but that can be avoided by
-fno-schedule-insns2 or by applying my proposed patch for PR38644.)

[Bug rtl-optimization/45813] [4.4/4.5 Regression] alias analysis problem with -mthumb

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |alias, wrong-code
             Target|ARM/Thumb                   |arm-elf, arm-linux-gnueabi
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|2010-09-28 09:17:33         |2010.09.30 09:56:46
               date|                            |
   Target Milestone|---                         |4.4.5
            Summary|alias analysis problem ?    |[4.4/4.5 Regression] alias
                   |                            |analysis problem with
                   |                            |-mthumb
     Ever Confirmed|0                           |1

--- Comment #11 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-09-30 09:56:46 UTC ---
Thus, confirmed.

[Bug rtl-optimization/45813] [4.4/4.5 Regression] alias analysis problem with -mthumb

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

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

[Bug rtl-optimization/45813] [4.4/4.5 Regression] alias analysis problem with -mthumb

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

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

[Bug rtl-optimization/45813] [4.4/4.5 Regression] alias analysis problem with -mthumb

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

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

[Bug rtl-optimization/45813] [4.4/4.5 Regression] alias analysis problem with -mthumb

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bernds at gcc dot gnu.org
      Known to work|                            |4.3.5, 4.6.0

--- Comment #12 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-19 09:06:03 UTC ---
The quoted revision fixing this for 4.6 doesn't look like a wrong-code fix
but a cost adjustment.

[Bug rtl-optimization/45813] [4.5 Regression] alias analysis problem with -mthumb

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.7                       |4.5.4

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-03-13 12:47:13 UTC ---
4.4 branch is being closed, moving to 4.5.4 target.

[Bug rtl-optimization/45813] [4.5 Regression] alias analysis problem with -mthumb

[bernds at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

Bernd Schmidt <bernds at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|bernds at gcc dot gnu.org   |

--- Comment #14 from Bernd Schmidt <bernds at gcc dot gnu.org> 2012-03-13 14:47:02 UTC ---
This really just needs -fno-strict-aliasing, doesn't it? Removing myself from
Cc.

[Bug rtl-optimization/45813] [4.5 Regression] alias analysis problem with -mthumb

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

--- Comment #15 from Mikael Pettersson <mikpe at it dot uu.se> 2012-03-15 08:10:55 UTC ---
-fno-strict-aliasing makes no difference to the code generated by 4.5-20120308.
 Replacing the type-punning with a union results in both correct-looking and
much tighter code:

unsigned short ReadLE16U_union( volatile unsigned char * ptr )
{
    union {
        unsigned short value;
        unsigned char bytes[2];
    } u;

    u.bytes[0] = ptr[0];
    u.bytes[1] = ptr[1];

    return u.value;
}

ReadLE16U_union:
        push    {r7, lr}
        ldrb    r3, [r0]
        ldrb    r0, [r0, #1]
        add     r7, sp, #0
        mov     sp, r7
        lsl     r0, r0, #8
        orr     r0, r0, r3
        @ sp needed for prologue
        pop     {r7, pc}
        .size   ReadLE16U_union, .-ReadLE16U_union
        .ident  "GCC: (GNU) 4.5.4 20120308 (prerelease)"

[Bug rtl-optimization/45813] [4.5 Regression] alias analysis problem with -mthumb

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45813

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|4.5.4                       |4.6.0

--- Comment #16 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-07-02 10:46:12 UTC ---
Fixed in 4.6.0, the 4.5 branch is being closed.