[Bug target/59371] [4.8/4.9 Regression] Performance regression in GCC 4.8 and later versions.

[macro@linux-mips.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59371

--- Comment #10 from Maciej W. Rozycki <macro@linux-mips.org> ---
(In reply to Jakub Jelinek from comment #9)

Jakub,

 The fix has corrected the evaluation of `i++' however it has regressed
the evaluation of `i < c'.  This is because in the loop `i' is only ever
assigned values that are lower than or equal to the value of `c' and
here the `int' type can represent all values representable with the
`signed short' and `unsigned short' types.

 Specifically, assuming the properties of the MIPS target, where `int' is
32-bit wide and `short' is 16-bit wide, we have the following cases of
the `c' input value here:

1. 0 -- the loop is not entered because `i' is preset to 0 and equals `c'
   right away.

2. [1,32767] -- `i' is incremented from 0 until it reaches the value of
   `c', at which point the loop terminates.

3. [32768,65535] -- `i' is incremented from 0 up to 32767, at which point
   it wraps around to -32768 and continues being incremented to 32767
   again.  And so on, and so on.  It never reaches the value of `c' or
   any higher value and therefore the loop never terminates.

Based on the above observations it is enough to check for `i == c' as the
loop termination condition.

 So I think this is still a performance regression from the user's point
of view even though I realise this may not necessarily be an optimisation
GCC has been previously designed for.  Therefore I'd prefer to keep the
bug open until/unless we decide it's impractical to implement a code
transformation that would restore previous performance.

  Maciej