Hi,
for tracing and logging one could easily write:
foo
...
Log enabled ifTrue: [ "do some tracing here" ].
...
but this always requires some message sends/checks if the debug mode is active.
This is OK while debugging/tracing to find errors - but for runtime one whishes
to have the
best performance without any effect of tracing/debug code.
Smalltalk/MT had a nice optimization feature that allowed to mix in debug code
without
additional overhead when in non-debug mode. If I remember correctly the basic
idea was the
following: there is a pool constant _DEBUG_INTERNAL that you compare to another
pool constant
TRUE (representing the value 1).
So with an expresion like _DEBUG_INTERNAL == TRUE this ends up in 1 == 1
comparision
which is always true if the pool constants/values have the same value of 1.
A followup message send of #ifTrue: will always be executed - therefore means
for the compiler
to included the (debug) code:
foo
...
_DEBUG_INTERNAL == TRUE ifTrue: [
Processor outputDebugLine: 'Trace something'
].
...
So in this case (always true) this was optimized by the compiler to
foo
...
Processor outputDebugLine: 'Trace something'
...
When _DEBUG_INTERNAL was set to FALSE (represented by value 0 as in C/C++) this
ends up in 0 == 1 which is always false and will never be true. So a followup
message
send of #ifTrue: was optimized (removed as it was dead code and never reached).
foo
...
...
So depending on pool flag _DEBUG_INTERNAL and recompilation of (all) methods one
could add debug/trace/logging code without runtime overhead. Yes this is very C
language
like to have conditional compilation - but very effective in keeping
debug/tracing
code influence low.
I know we usually have the debugger in front of us - but sometimes a trace or
log is
required to see where code crashes. Think of a headless situation in a
webserver or
a small device like the pi where you want to reduce the performance overhead.
This leads to several questions:
- Is something like this feasible/already possible in Pharo?
- Any pointers on how Opal does optimizations? Do we have similar optimizations
(removing
code that could not be reached) that could be used.
- How do you usually deal with additional tracing code when you do not want to
have too much runtime overhead.
Thanks
T.
