[Bug tdep/26000] New: -Wtautological-bitwise-compare error in arm-tdep.c

[Bug tdep/26000] New: -Wtautological-bitwise-compare error in arm-tdep.c

[simark at simark dot ca]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

            Bug ID: 26000
           Summary: -Wtautological-bitwise-compare error in arm-tdep.c
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: simark at simark dot ca
  Target Milestone: ---

We get this error in arm-tdep.c when building with clang 11:

  CXX    arm-tdep.o
/home/smarchi/src/binutils-gdb/gdb/arm-tdep.c:934:29: error: bitwise comparison
always evaluates to false [-Werror,-Wtautological-bitwise-compare]
          else if ((insn & 0xffb0) == 0xe950    /* ldrd Rt, Rt2,
                   ~~~~~~~~~~~~~~~~^~~~~~~~~

Indeed, this can never evaluate to true.

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[simark at simark dot ca]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

Simon Marchi <simark at simark dot ca> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |luis.machado at linaro dot org

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[luis.machado at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

Luis Machado <luis.machado at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

--- Comment #1 from Luis Machado <luis.machado at linaro dot org> ---
As was discussed on IRC, Simon hinted at problems handling these ldrd/strd's.

Even when fixing the pattern to match ldrd (immediate), with mask 0xff50, the
mask isn't strict enough to match *only* ldrd (immediate). It will also match
ldrd (literal). So we need to verify op1 != 1111 to rule out ldrd (literal).

Then there is the question of whether we want to handle all variants of
ldrd/strd's, which may or may not be important to fix.

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[simark at simark dot ca]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

--- Comment #2 from Simon Marchi <simark at simark dot ca> ---
(In reply to Luis Machado from comment #1)
> As was discussed on IRC, Simon hinted at problems handling these ldrd/strd's.
> 
> Even when fixing the pattern to match ldrd (immediate), with mask 0xff50,
> the mask isn't strict enough to match *only* ldrd (immediate). It will also
> match ldrd (literal). So we need to verify op1 != 1111 to rule out ldrd
> (literal).
> 
> Then there is the question of whether we want to handle all variants of
> ldrd/strd's, which may or may not be important to fix.

To clarify, could you show here what's the full condition you would use with
mask 0xff50?

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[luis.machado at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

--- Comment #3 from Luis Machado <luis.machado at linaro dot org> ---
Created attachment 12548
  --> https://sourceware.org/bugzilla/attachment.cgi?id=12548&action=edit
Tentative patch

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[luis.machado at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

--- Comment #4 from Luis Machado <luis.machado at linaro dot org> ---
This is what i plan to submit as a fix, after i'm done with testing.

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[luis.machado at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

Luis Machado <luis.machado at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #12548|0                           |1
        is obsolete|                            |

--- Comment #5 from Luis Machado <luis.machado at linaro dot org> ---
Created attachment 12549
  --> https://sourceware.org/bugzilla/attachment.cgi?id=12549&action=edit
Tentative patch

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

--- Comment #6 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Luis Machado <luisgpm@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=f8c6d1528c19b11fdaa3ec949147280e500446e2

commit f8c6d1528c19b11fdaa3ec949147280e500446e2
Author: Luis Machado <luis.machado@linaro.org>
Date:   Fri May 15 23:06:52 2020 -0300

    Fix PR 26000, logical bitwise error / prologue analyzer

    This fixes an instruction mask typo. We should be matching only
    ldrd (immediate) and not any other of its variants. As is, it never matches
    anything.

    With the patch, the instruction mask also allows matching of ldrd
(literal),
    but the check for SP discards this particular instruction pattern, as it
has
    a hardcoded PC register.

    gdb/ChangeLog:

    2020-05-27  Luis Machado  <luis.machado@linaro.org>

            PR tdep/26000
            * arm-tdep.c (thumb_analyze_prologue): Fix instruction matching
            for ldrd (immediate).

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[Bug tdep/26000] -Wtautological-bitwise-compare error in arm-tdep.c

[luis.machado at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=26000

Luis Machado <luis.machado at linaro dot org> changed:

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

--- Comment #7 from Luis Machado <luis.machado at linaro dot org> ---
Fixed with the last patch.

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