Date: Sun, 15 Jan 1995 11:50:03 +0900
From: Stephen Turnbull <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
To: drupp AT cs DOT washington DOT edu
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: gcc = gcc -O2 ?

   From: drupp AT cs DOT washington DOT edu (Douglas Rupp)

   Marin Oldfield writes:
   "I think that changing the default optimization level as a side-effect
   of -g is dangerous and counterintuitive." 

   -------

   I don't understand your reasoning here.  It seems to me that turning off
   all optimization when debugging (by default) is exactly what you would
   expect and want.

The point is that we want flags to be orthogonal in their operation.
It is true that at the moment it is relatively hard to (source-level)
debug optimized code (temporary variables appear and disappear,
references to named variables can disappear, etc), but this can fixed
by better compiler and debugger technology.

The problem is that optimization introduces changes in the code which
can expose bugs or nonportable practices.  For one example, literal
strings should be considered read-only, because a certain amount of
space can be saved by making the (hidden) pointers point to common
string storage where tails of strings are the same.  As far as I know,
GNU C always does this optimization, but let's suppose that it only
does it for optimization levels greater than 1.  Consider a program
which has a char* variable initialized by a string literal, and as a
space optimization changes a couple of characters.  (Eg, the string is
"Press \"Y\" to continue: ", and depending on the context of the
preceding text we might want the trigger key to be 'N'.)  Then with
the "-g => -O0" scheme, we compile with "-g", debug, and have no
problems.  So now we compile without "-g" to save the disk space, and
if that string appears several times in a file, the compiler would
optimize the storage.  Those uses that do *not* change the string's
value will get the wrong string.  OK, we have a bug, so we turn on "-g"
to debug, and the bug disappears.  This is what Martin meant by
"dangerous".

Yes, if you read the manual carefully, you will know that you should
turn on "-O0".  The juxtaposition of "turn on" with the "optimization
off" flag is intentional.  This is what Martin meant by
"counterintuitive."

    --Steve