Other Compilation Options

Many combinations of compiler options work for LoopTool and LoopReport.

To compile for automatic parallelization, typical compilation switches are -xparallel and -x04. To compile for LoopTool and LoopReport, add -Zlp.

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% f77 -x04 -xparallel -Zlp source_file

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You can use either -xO3 or -xO4 with -xparallel. If you don't specify -xO3 or -xO4 but you do use -xparallel, then the compiler uses -xO3. Table 3-1 summarizes how optimization level options are added for specific options.

Table 3-1 Optimization Level Options and What They Imply

You type:	Bumped Up To:
-xparallel	-xparallel -xO3
-xparallel -Zlp	-xparallel -xO3 -Zlp
-xexplicitpar	-xexplicitpar -xO3
-xexplicitpar -Zlp	-xexplicitpar -xO3 -Zlp
-Zlp	-xdepend -xO3 -Zlp

Other compilation options include -xexplicitpar and -xloopinfo.

The Fortran compiler option -xexplicitpar is used with the pragma DOALL. If you insert DOALL before a loop in your source code, you are explicitly marking that loop for parallelization. The compiler parallelizes the loop when you compile with -xexplicitpar.

The following code fragment shows how to mark a loop explicitly for parallelization.

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	subroutine adj(a,b,c,x,n)
	  real*8 a(n), b(n), c(-n:0), x
	    integer n
c$par DOALL
	do 19 i = 1, n*n
	  do 29 k = i, n*n
	    a(i) = a(i) + x*b(k)*c(i-k)
29	  continue
19	continue
	return
	end

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When you use -Zlp by itself, -xdepend and -xO3 are added. The -xdepend option instructs the compiler to perform the data dependency analysis that it needs to do to identify loops. The option -xparallel includes -xdepend, but -xdepend does not imply (or trigger) -xparallel.

The -xloopinfo option prints hints about loops to stderr (the UNIX standard error file, on file descriptor 2) when you compile your program. The hints include the routine names, the line number for the start of the loop, whether the loop was parallelized, and the reason it was not parallelized, if applicable.

The following example redirects hints about loops in the source file gamteb.F to the file gamtab.loopinfo:

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% f77 -xO3 -parallel -xloopinfo -Zlp gamteb.F 2> gamteb.loopinfo

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The main difference between -Zlp and -xloopinfo is that in addition to providing compiler hints about loops, -Zlp also instruments your program so that timing statistics are recorded at runtime. For this reason, also, LoopTool and LoopReport analyze only programs that have been compiled with -Zlp.