[flac-dev] Undefined behaviour

[Erik de Castro Lopo]

Hi all,

People watching the git commits might have noticed that I have been
fixing a number of issues around undefined behaviour.

Why you ask?

* Some forms of undefined behaviour have potential for security exploits.
* Compiler writers are free to replace anything which invokes UB with a
  NOP or even, nothing at all.
* Having large numbers of UB warnings makes it difficult (or rather time
  consuming) to check them all for the possibility that they are a
  potentially exploitable.

Most of these UB changes have little liklihood of performance regressions.
However, this one:

    commit 1b8af6bb45a9ad74fa374fb6414974e63ffc793b
    Author: Erik de Castro Lopo <erikd at mega-nerd.com>
    Date:   Sat Aug 29 05:21:43 2015 +1000

    libFLAC/fixed.c: Fix undefined behaviour
    
    Left shift if a negative integer such that the sign bit is affected is
    (according to the C spec) undefined behaviour and the residual
    calculations using the shift operator were hitting this.
    
    Fortunately these same calculations using plain multiplication do not
    invoke UB and according to benchmarking (on x86_64 linux) have the same
    performance as the bit shift version.


has no performance impact on x86_64 linux, *may* not be the same with
other compilers.

Since this code is in such an inner loop, any banchmark should show up
regressions if they exist. Please test.

Cheers,
Erik
-- 
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Erik de Castro Lopo
http://www.mega-nerd.com/