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is also used to describe the situation where, due to either a hardware design choice or a hardware failure, one or more of the available address bits is not used in the memory selection process. This may be a design decision if there are more address bits available than are necessary to support the
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can be accessed through different symbolic names in the program. Thus, modifying the data through one name implicitly modifies the values associated with all aliased names, which may not be expected by the programmer. As a result, aliasing makes it particularly difficult to understand, analyze and
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If this change occurred to a different address bit, the decoding results would be different, but in general the effect would be the same: the loss of a single address bit cuts the available memory space in half, with resulting duplication (aliasing) of the remaining space.
74:). This is a common problem with functions that accept pointer arguments, and their tolerance (or the lack thereof) for aliasing must be carefully documented, particularly for functions that perform complex manipulations on memory areas passed to them.
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edition, see section 6.5, paragraph 7) specifies that it is illegal (with some exceptions) to access the same memory location using pointers of different types. A compiler may therefore assume that such pointers do not alias. This rule, known as the
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In the table above, each of the 8 unique combinations of address bits selects a different memory location. However, if one address bit (say A2) were to be shorted to ground, the table would be modified as follows:
303:, sometimes allows for impressive increases in performance, but has been known to break some otherwise valid code. Several software projects intentionally violate this portion of the C99 standard. For example,
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In this case, with A2 always being zero, the first four memory locations are duplicated and appear again as the second four. Memory locations 4 through 7 have become inaccessible.
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Controlled aliasing behaviour may be desirable in some cases (that is, aliasing behaviour that is specified, unlike that enabled by memory layout in C). It is common practice in
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will print out "2 3 4" as a result. If one wanted to bypass aliasing effects, one could copy the contents of the index variable into another and change the copy.
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often have to make conservative assumptions about variables when aliasing is possible. For example, knowing the value of a variable (such as
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352:, since 2 = 8). Address bits (named A2 through A0) are decoded to select unique memory locations as follows, in standard
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does this because strict aliasing causes problems with optimization of inlined code. In such cases, when compiled with
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Aliasing can occur in any language that can refer to one location in memory with more than one name (for example, with
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204:). However, the compiler cannot use this information after an assignment to another variable (for example, in C,
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installed memory device(s). In a failure, one or more address bits may be shorted together, or may be forced to
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For this example, assuming a memory design with 8 locations, requiring only 3 address lines (or
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loops. This allows certain data structures to be modified directly with less code. For example,
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Another optimization impacted by aliasing is code reordering. If the compiler decides that
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intend to make and compute useful information for understanding aliasing in programs.
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for uses of the word when applied to signal processing, including computer graphics
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772:– article on strict aliasing originally from the boost developer's wiki
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For the term used in signals processing and computer graphics, see
156:# modifying @array, since $ element is ''aliased''
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would be changed as well, so propagating the information that
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specifies, in some constructs, aliasing behaviour, such as in
766:– Informational article on type-based alias analysis in C++
726:"Re: Invalid compilation without -fno-strict-aliasing"
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To enable such optimizations in a predictable manner,
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is 5) normally allows certain optimizations (such as
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216:. This could be the case after an assignment like
53:describes a situation in which a data location in
760:– informational article on NetBSD mailing list
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340:(logic 0) or the supply voltage (logic 1).
159:# to each of @array's elements in turn.
34:. For aliasing in statistical design, see
153:# Increment $ element, thus automatically
27:Multiple names for the same data location
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220:. As an effect of this assignment to
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758:Aliasing, pointer casts and gcc 3.3
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711:"ANSI strict aliasing and Python"
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770:Understand C/C++ Strict Aliasing
764:Type-based alias analysis in C++
36:Aliasing (factorial experiments)
741:"Software Based Memory Testing"
709:Neil Schemenauer (2003-07-17).
696:"Understanding Strict Aliasing"
236:would be potentially wrong (if
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724:Linus Torvalds (2003-02-26).
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256:can be propagated safely.
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694:Mike Acton (2006-06-01).
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38:. For the command, see
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294:(including its newer
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780:Categories
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305:Python 2.x
277:scheduling
218:y = &x
194:Optimizers
356:fashion:
331:The term
162:$ element
138:$ element
47:computing
674:Aliasing
663:See also
333:aliasing
290:for the
72:pointers
51:aliasing
32:Aliasing
344:Example
273:*y = 10
234:*y = 10
206:*y = 10
132:foreach
95:foreach
84:Fortran
338:ground
144:@array
105:@array
86:. The
55:memory
254:x = 5
174:print
350:bits
88:Perl
317:gcc
296:C99
45:In
782::
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519:0
502:A0
499:A1
496:A2
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385:0
368:A0
365:A1
362:A2
265:*y
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238:*y
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210:*y
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135:my
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102:my
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