At optimization level +O0, the
compiler performs the following
optimizations that span no more than a single source statement:
Partial evaluation of test conditions
Simple register assignment
Data alignment on natural boundaries
The default optimization level is +O0.
Constant folding |
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Constant folding is the replacement of operations on constants
with the result of the operation. For example, Y=5+7
is replaced with Y=12.
More advanced constant folding is performed at optimization
level +O2. See the section “Advanced constant folding and propagation” for more information.
Partial evaluation of test conditions |
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Where possible, the compiler determines the truth value of a logical
expression without evaluating all the operands (also known as
short-circuiting). Consider the Fortran example below:
IF ((I .EQ. J) .OR. (I .EQ. K)) GOTO 100
If (I .EQ. J) is true, control
immediately goes to 100; otherwise,
(I .EQ. K) must be evaluated before
control can go to 100 or the following
statement.
Do not rely upon partial evaluation if you use function calls
in the logical expression because:
There is no guarantee on the order
of evaluation.
A procedure or function call can have side effects
on variable values that may or may not be partially evaluated correctly.
Simple register assignment |
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The compiler may place frequently used variables in registers to
avoid more costly accesses to memory.
A more advanced register assignment algorithm is used at optimization
level +O2. See the section “Global register allocation” for more information.
Data alignment on natural boundaries |
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The compiler automatically aligns data objects to their natural
boundaries in memory,
providing more
efficient access to data. This means that a data object's
address is integrally divisible by the length of its data type;
for example, REAL*8 objects have
addresses integrally divisible by 8 bytes.
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 | NOTE: Aliases can inhibit data alignment. Be especially careful
when equivalencing arrays in Fortran. |
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You should declare scalar variables in order from longest
to shortest data length to ensure the efficient layout of such aligned
data in memory. This minimizes the amount of padding the compiler
has to do to get the data onto its natural boundary.
Consider the following Fortran example:
C CAUTION: POORLY ORDERED DATA FOLLOWS:
LOGICAL*2 BOOL
INTEGER*8 A, B
REAL*4 C
REAL*8 D |
Here, the compiler must insert 6 blank bytes after BOOL
in order to correctly align A,
and it must insert 4 blank bytes after C
to correctly align D.
The same data is more efficiently ordered as shown in the
following example:
C PROPERLY ORDERED DATA FOLLOWS:
INTEGER*8 A, B
REAL*8 D
REAL*4 C
LOGICAL*2 BOOL |
Natural boundary alignment is performed on all data. Do not
confuse It with cache line boundary alignment, which is performed
as described in the section “Data alignment”. Also discussed in Chapter 2 are the align_cti
directive and pragma, which facilitate CTIcache line boundary alignment.
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