At optimization level +O0,
the compiler performs optimizations that span only a single source
statement. This is the default. The +O0
machine instruction level optimizations include:
Partial evaluation of test conditions
Simple register assignment
Data alignment on natural boundaries
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. This is
known as short-circuiting. The Fortran example below describes this:
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 (GRA)” 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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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.
Data alignment on natural boundaries
The following Fortran example describes the alignment of data
objects to their natural boundaries:
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 unused bytes after BOOL
in order to correctly align A,
and it must insert 4 unused 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. This
is not to be confused with cache line boundary alignment.
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