[Bug exp/18617] New: Incorrect expression bytecode generated for narrowing conversions

[Bug exp/18617] New: Incorrect expression bytecode generated for narrowing conversions

[robert at ocallahan dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

            Bug ID: 18617
           Summary: Incorrect expression bytecode generated for narrowing
                    conversions
           Product: gdb
           Version: 7.9
            Status: NEW
          Severity: normal
          Priority: P2
         Component: exp
          Assignee: unassigned at sourceware dot org
          Reporter: robert at ocallahan dot org
  Target Milestone: ---

Given the program:

uint64_t u64max = (uint64_t)(int64_t)-1;
int main(int argc, char* argv[]) {
  return 0;
}

and the commands

break main
cond 1 (unsigned char)u64max==255

the following expression bytecode is generated:

{0x24, 0x0, 0x60, 0xd, 0x38, 0x1a, 0x2a, 0x40, 0x23, 0x0, 0xff, 0x13, 0x27}

That is:

  const32 0x600d38
  ref64
  zero_ext 64
  const16 0xff
  equal
  end

The zero_ext operand is incorrect. It should have been 8, to keep the low 8
bits and zero the rest. The breakpoint condition therefore returns false when
it should return true.

The bug is in ax-gdb.c:

  /* If we're converting to a narrower type, then we need to clear out
     the upper bits.  */
  if (TYPE_LENGTH (to) < TYPE_LENGTH (from))
    gen_extend (ax, from);

That should be
    gen_extend (ax, to);
to keep `to` bits and zero the rest.

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[khuey at kylehuey dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

Kyle Huey (Mozilla) <khuey at kylehuey dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |khuey at kylehuey dot com

--- Comment #1 from Kyle Huey (Mozilla) <khuey at kylehuey dot com> ---
Your testcase WFM on gdb 7.7.

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[robert at ocallahan dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

Robert O'Callahan <robert at ocallahan dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |robert at ocallahan dot org

--- Comment #2 from Robert O'Callahan <robert at ocallahan dot org> ---
Created attachment 8406
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8406&action=edit
preserve 'to' bits when generating bytecode for a narrowing conversion

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[palves at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

Pedro Alves <palves at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |palves at redhat dot com

--- Comment #3 from Pedro Alves <palves at redhat dot com> ---
Hi.  Thanks for finding and fixing this.

It'd be great if you sent this as a full/finished patch to the patches list.
Then someone can git am/push it.

Please take a look at contribution guidelines in the wiki:
  https://sourceware.org/gdb/wiki/ContributionChecklist

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[robert at ocallahan dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

--- Comment #4 from Robert O'Callahan <robert at ocallahan dot org> ---
https://sourceware.org/ml/gdb-patches/2015-07/msg00089.html

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

--- Comment #5 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Pedro Alves <palves@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=bcf5c1d96b3b5493041002eb2b6d27288d9d160a

commit bcf5c1d96b3b5493041002eb2b6d27288d9d160a
Author: Robert O'Callahan <robert@ocallahan.org>
Date:   Wed Jul 8 10:53:36 2015 +0100

    PR18617 - Incorrect expression bytecode generated for narrowing conversions

    The existing code preserves 'from' bits, which is incorrect.  E.g.

     (gdb) maint agent-eval (char)255L
     Scope: 0x4008d6
     Reg mask: 00
       0  const16 255
       3  ext 64
       5  end

    'ext 64' should be 'ext 8'; this bytecode evaluates to 255 instead of
    the correct result of -1.  The fix is simple.  I ran the entire test
    suite on x86-64 and there were no new test failures.

    gdb/ChangeLog:
    2015-07-08  Robert O'Callahan  <robert@ocallahan.org>

        PR exp/18617
        * ax-gdb.c (gen_conversion): Extend to 'to' bits, not 'from'.

    gdb/testsuite/ChangeLog:
    2015-07-08  Robert O'Callahan  <robert@ocallahan.org>

        PR exp/18617
        * gdb.trace/ax.exp: Add test.

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

--- Comment #6 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The gdb-7.10-branch branch has been updated by Pedro Alves
<palves@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=cc1259417e727c47e58cea1bb4a148974689ad8e

commit cc1259417e727c47e58cea1bb4a148974689ad8e
Author: Robert O'Callahan <robert@ocallahan.org>
Date:   Wed Jul 8 11:11:22 2015 +0100

    PR18617 - Incorrect expression bytecode generated for narrowing conversions

    The existing code preserves 'from' bits, which is incorrect.  E.g.

     (gdb) maint agent-eval (char)255L
     Scope: 0x4008d6
     Reg mask: 00
       0  const16 255
       3  ext 64
       5  end

    'ext 64' should be 'ext 8'; this bytecode evaluates to 255 instead of
    the correct result of -1.  The fix is simple.  I ran the entire test
    suite on x86-64 and there were no new test failures.

    gdb/ChangeLog:
    2015-07-08  Robert O'Callahan  <robert@ocallahan.org>

        PR exp/18617
        * ax-gdb.c (gen_conversion): Extend to 'to' bits, not 'from'.

    gdb/testsuite/ChangeLog:
    2015-07-08  Robert O'Callahan  <robert@ocallahan.org>

        PR exp/18617
        * gdb.trace/ax.exp: Add test.

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[Bug exp/18617] Incorrect expression bytecode generated for narrowing conversions

[palves at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=18617

Pedro Alves <palves at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |7.10

--- Comment #7 from Pedro Alves <palves at redhat dot com> ---
Fixed.

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