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Top Forums Programming Memory and cache access time discrepancy Post 302928318 by achenle on Wednesday 10th of December 2014 09:42:16 PM
Old 12-10-2014
The compiler's optimizer noted that the value of "r" was never used, and eliminated the operation.

And if the code isn't optimized, the compiler almost certainly produced a LOT more instructions than necessary to simply load a value from memory into a register.

Nevermind the time spend actually doing the rdtsc() routines themselves.
 

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CC(1)							      General Commands Manual							     CC(1)

NAME
cc, pcc - C compiler SYNOPSIS
cc [ option ] ... file ... pcc [ option ] ... file ... DESCRIPTION
Cc is the UNIX C compiler. It accepts several types of arguments: Arguments whose names end with `.c' are taken to be C source programs; they are compiled, and each object program is left on the file whose name is that of the source with `.o' substituted for `.c'. The `.o' file is normally deleted, however, if a single C program is compiled and loaded all at one go. In the same way, arguments whose names end with `.s' are taken to be assembly source programs and are assembled, producing a `.o' file. The following options are interpreted by cc. See ld(1) for load-time options. -c Suppress the loading phase of the compilation, and force an object file to be produced even if only one program is compiled. -p Arrange for the compiler to produce code which counts the number of times each routine is called; also, if loading takes place, replace the standard startup routine by one which automatically calls monitor(3) at the start and arranges to write out a mon.out file at normal termination of execution of the object program. An execution profile can then be generated by use of prof(1). -f In systems without hardware floating-point, use a version of the C compiler which handles floating-point constants and loads the object program with the floating-point interpreter. Do not use if the hardware is present. -O Invoke an object-code optimizer. -S Compile the named C programs, and leave the assembler-language output on corresponding files suffixed `.s'. -P Run only the macro preprocessor and place the result for each `.c' file in a corresponding `.i' file and has no `#' lines in it. -E Run only the macro preprocessor and send the result to the standard output. The output is intended for compiler debugging; it is unacceptable as input to cc. -o output Name the final output file output. If this option is used the file `a.out' will be left undisturbed. -Dname=def -Dname Define the name to the preprocessor, as if by `#define'. If no definition is given, the name is defined as 1. -Uname Remove any initial definition of name. -Idir `#include' files whose names do not begin with `/' are always sought first in the directory of the file argument, then in directo- ries named in -I options, then in directories on a standard list. -Bstring Find substitute compiler passes in the files named string with the suffixes cpp, c0, c1 and c2. If string is empty, use a standard backup version. -t[p012] Find only the designated compiler passes in the files whose names are constructed by a -B option. In the absence of a -B option, the string is taken to be `/usr/c/'. Other arguments are taken to be either loader option arguments, or C-compatible object programs, typically produced by an earlier cc run, or perhaps libraries of C-compatible routines. These programs, together with the results of any compilations specified, are loaded (in the order given) to produce an executable program with name a.out. The major purpose of the `portable C compiler', pcc, is to serve as a model on which to base other compilers. Pcc does not support options -f, -E, -B, and -t. It provides, in addition to the language of cc, unsigned char type data and initialized bit fields. FILES
file.c input file file.o object file a.out loaded output /tmp/ctm? temporaries for cc /lib/cpp preprocessor /lib/c[01] compiler for cc /usr/c/oc[012] backup compiler for cc /usr/c/ocpp backup preprocessor /lib/fc[01] floating-point compiler /lib/c2 optional optimizer /lib/crt0.o runtime startoff /lib/mcrt0.o startoff for profiling /lib/fcrt0.o startoff for floating-point interpretation /lib/libc.a standard library, see intro(3) /usr/include standard directory for `#include' files /tmp/pc* temporaries for pcc /usr/lib/ccom compiler for pcc SEE ALSO
B. W. Kernighan and D. M. Ritchie, The C Programming Language, Prentice-Hall, 1978 D. M. Ritchie, C Reference Manual monitor(3), prof(1), adb(1), ld(1) DIAGNOSTICS
The diagnostics produced by C itself are intended to be self-explanatory. Occasional messages may be produced by the assembler or loader. Of these, the most mystifying are from the assembler, as(1), in particular `m', which means a multiply-defined external symbol (function or data). BUGS
Pcc is little tried on the PDP11; specialized code generated for that machine has not been well shaken down. The -O optimizer was designed to work with cc; its use with pcc is suspect. PDP11 CC(1)
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