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[Qemu-devel] [Bug 1655700] [NEW] disas/libvixl/vixl/invalset.h: possible


From: dcb
Subject: [Qemu-devel] [Bug 1655700] [NEW] disas/libvixl/vixl/invalset.h: possible dodgy code in binary search ?
Date: Wed, 11 Jan 2017 16:06:31 -0000

Public bug reported:


[qemu/disas/libvixl/vixl/invalset.h:442]: (style) Array index 'low' is used 
before limits check.

Source code is

    while (!IsValid(elements[low]) && (low < high)) ++low;

Also:

qemu/disas/libvixl/vixl/invalset.h:450]: (style) Array index 'middle' is
used before limits check.

The source code is

   while (!IsValid(elements[high]) && (low < high)) --high;

Mind you, these lines of code look similar but didn't get reported:

    while (!IsValid(elements[middle]) && (middle < high - 1)) ++middle;
    while (!IsValid(elements[middle]) && (low + 1 < middle)) --middle;

Given that binary search is notoriously tricky to get correct and a standard C 
library routine
I am puzzled as to why the standard library routine didn't get used, with of 
course a custom
comparison function.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1655700

Title:
  disas/libvixl/vixl/invalset.h: possible dodgy code in binary search ?

Status in QEMU:
  New

Bug description:
  
  [qemu/disas/libvixl/vixl/invalset.h:442]: (style) Array index 'low' is used 
before limits check.

  Source code is

      while (!IsValid(elements[low]) && (low < high)) ++low;

  Also:

  qemu/disas/libvixl/vixl/invalset.h:450]: (style) Array index 'middle'
  is used before limits check.

  The source code is

     while (!IsValid(elements[high]) && (low < high)) --high;

  Mind you, these lines of code look similar but didn't get reported:

      while (!IsValid(elements[middle]) && (middle < high - 1)) ++middle;
      while (!IsValid(elements[middle]) && (low + 1 < middle)) --middle;

  Given that binary search is notoriously tricky to get correct and a standard 
C library routine
  I am puzzled as to why the standard library routine didn't get used, with of 
course a custom
  comparison function.

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