[fpc-devel] Optimizer Overhaul Take 2... Jump Optimizations

[J. Gareth Moreton]

A partial example of where "condition_in" shows its power (compiled code 
from fpc\packages\fcl-base\src\advancedipc.pp)

BEFORE:

     ...
     jb    .Lj178
     subl    $17,%eax
     cmpl    $26,%eax
     jb    .Lj178
     cmpl    $30,%eax
     stc
     je    .Lj178
     subl    $32,%eax
     cmpl    $26,%eax
.Lj178:
     jc    .Lj180    (Appears later in the code)
     ...

AFTER:

     ...
     jb    .Lj180    <-- Label changed
     subl    $17,%eax
     cmpl    $26,%eax
     jb    .Lj180    <-- Label changed
     cmpl    $30,%eax
     stc
     je    .Lj178
     subl    $32,%eax
     cmpl    $26,%eax
.Lj178:
     jc    .Lj180    (Appears later in the code)
     ...

In this case, 'condition_in' detects that C_C is a subset of C_B 
(they're actually equivalent in this case... both branch if the Carry 
Flag is set).  This particular optimisation means that the code only has 
to branch once if the JB conditions are met, rather than twice.

Gareth aka. Kit


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