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* cleaned up/corrected some parts about optimizations

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Jonas Maebe 1998-10-26 21:07:29 +00:00
parent c4bc24c00b
commit 4ae9c1291d
1 changed files with 24 additions and 28 deletions
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docs/prog.tex
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@ -2927,7 +2927,7 @@ one copy of the string constant.
Evaluation of boolean expression stops as soon as the result is
known, which makes code execute faster then if all boolean operands
were evaluted.
were evaluated.
\subsection{ Constant set inlining }
@ -3010,15 +3010,15 @@ implemented until version 0.99.6 of \fpc.
\subsection{ Case optimization }
When using the \var{-O1} switch, case statements in certain cases will
be decoded using a jump table, which in certain cases will make the
case statement execute faster.
When using the \var{-O1} (or higher) switch, case statements will be
generated using a jump table if appropriate, to make them execute
faster.
\subsection{ Stack frame omission }
Under certain specific conditions, the stack frame (entry and exit code
for the routine, see section \ref{se:Calling}) will be omitted, and
the variable will directly be accessed via the stack pointer.
Under specific conditions, the stack frame (entry and exit code for
the routine, see section \ref{se:Calling}) will be omitted, and the
variable will directly be accessed via the stack pointer.
Conditions for omission of the stack frame :
@ -3049,31 +3049,29 @@ the following is done:
\begin{itemize}
\item In \var{case} statements, a check is done whether a jump table
or a sequence of conditional jumps should be used for optimal performance.
\item Determines a number of strategies when doing peephole optimization:
\var{movzbl (\%ebp), \%eax} on PentiumPro and PII systems will be changed
into \var{xorl \%eax,\%eax; movb (\%ebp),\%al } for lesser systems.
\item Determines a number of strategies when doing peephole optimization, e.g.:
\var{movzbl (\%ebp), \%eax} will be changed into
\var{xorl \%eax,\%eax; movb (\%ebp),\%al } for Pentium and PentiumMMX.
\end{itemize}
Cyrix \var{6x86} processor owners should optimize with \var{-Op3} instead of
\var{-Op2}, because \var{-Op2} leads to larger code, and thus to smaller
speed, according to the Cyrix developers FAQ.
\item When optimizing for speed (\var{-OG}, the default) or size (\var{-Og}), a choice is
\item When optimizing for speed (\var{-OG}, the default) or size (\var{-Og}), a choice is
made between using shorter instructions (for size) such as \var{enter \$4},
or longer instructions \var{subl \$4,\%esp} for speed. When smaller size is
requested, things aren't aligned on 4-byte boundaries. When speed is
requested, things are aligned on 4-byte boundaries as much as possible.
\item Simple optimization (\var{-O1}) makes sure the peephole optimizer is
used, as well as the reloading optimizer.
\item Uncertain optimizations (\var{-Ou}): With this switch, the reloading
optimizer can be forced into making uncertain
optimizations.
\item Fast optimizations (\var{-O1}): activate the peephole optimizer
\item Slower optimizations (\var{-O2}): also activate the common subexpression
elimination (formaerly called the "reloading optimizer)
\item Uncertain optimizations (\var{-Ou}): With this switch, the common subexpression
elimination algorithm can be forced into making uncertain optimizations.
Although you can enable uncertain optimizations in most cases, for people who
do not understand the follwong technical explanation, it might be the safes to
leave them off.
You can enable uncertain optimizations only in certain cases,
otherwise you will produce a bug; the following technical description
tells you when to use them:
\begin{quote}
% Jonas's own words..
\em
If uncertain optimizations are enabled, the reloading optimizer assumes
If uncertain optimizations are enabled, the CSE algortihm assumes
that
\begin{itemize}
\item If something is written to a local/global register or a
@ -3086,10 +3084,8 @@ procedure/function parameter.
% end of quote
\end{quote}
The practical upshot of this is that you cannot use the uncertain
optimizations if you access any local or global variables through pointers. In
theory, this includes \var{Var} parameters, but it is all right
if you don't both read the variable once through its \var{Var} reference
and then read it using it's name.
optimizations if you both write and read local or global variables directly and
through pointers (this includes \var{Var} parameters, as those are pointers too).
The following example will produce bad code when you switch on
uncertain optimizations:
@ -3147,7 +3143,7 @@ Begin
End.
\end{verbatim}
Will produce correct code, because the global variable \var{MyRecArrayPtr}
is not accessed directly, but through a pointer (\var{MyRecPtr} in this
is not accessed directly, but only through a pointer (\var{MyRecPtr} in this
case).
In conclusion, one could say that you can use uncertain optimizations {\em