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From: dalegass@dalcsug.UUCP
Newsgroups: comp.sys.ibm.pc,comp.lang.c
Subject: Turbo C 'optimization'
Message-ID: <126@dalcsug.UUCP>
Date: Sat, 15-Aug-87 09:26:53 EDT
Article-I.D.: dalcsug.126
Posted: Sat Aug 15 09:26:53 1987
Date-Received: Sun, 16-Aug-87 09:33:39 EDT
Organization: Dalhousie University, Halifax, N.S., Canada
Lines: 59
Keywords: Is it any good?
Xref: utgpu comp.sys.ibm.pc:5922 comp.lang.c:3499

Does anyone have views regarding the quality of code generated by Turbo C?
It claims to be one of the best around, but I've seen some code generated
that I consider rather disappointing.  

I think my main gripe would be the code generated by the basic multiply
and divide operations.  For example, consider the following code fragment:

long long_mult(int i, int j)
{
long k;

k = (long) i * j;
return k
}

Whereas Aztec C would produce code something along the lines of:
	MOV	AX,_i
	MUL	_j

Turbo C generates something like this:
	MOV	AX,_i
	CWD
	PUSH	DX
	PUSH	AX
	POP	BX
	POP	CX
	PUSH	CS
	CALL	LXMUL@

I consider this to be a pretty basic requirement, and doing a call for
a multiply is a bit much, in my view.  Even in long*long=long multiplications,
Aztec C simply puts a couple of IMUL's and ADDs inline.

Similarly, I would like 

int i,j;
long k;

i = (int) (k / j)

to simply use the IDIV instruction once, rather than a call, as Turbo C does.


This poor code generation (and resulting speed degredation) has led me to 
manually code time-critical parts of my programs into machine language, 
which I don't think should have been necessary (and isn't in Aztec C).

One thing Turbo C does, that I consider to be important optimization is 
the way the peek() and poke() calls are translated into direct memory reads
and writes, if you #include <dos.h>.  This can really speed things up.

Don't get me wrong: I think Turbo C is the best C development environment
available, and I constantly use and enjoy the package.

Hopefully Version 2.0 will do a little bit better optimization...



-dalegass@dalcsug.uucp