LNO(5) Last changed: 3-10-98
LNO - Compiler loop nest optimization option group
-LNO: ...
IRIX systems
This man page describes the loop nest optimization options accepted by
the f90(1), f77(1), CC(1), cc(1), and c89(1) commands.
The -LNO: option group specifies options and transformations performed
on loop nests. The -LNO: option group is enabled only if the -O3
option is also specified on the compiler command line.
For information on the LNO options that are in effect during a
compilation, use the -LIST:options=ON option.
You can specify more than one suboption to the -LNO: option either by
using colons to separate each suboption or by specifying multiple
options on the command line. For example, the following command lines
are equivalent:
f90 -LNO:auto_dist=ON:outer=OFF b.f
f90 -LNO:auto_dist=ON -LNO:outer=OFF b.f
Some -LNO: suboptions are specified with a setting that either enables
or disables the feature. To enable a feature, specify the argument
either alone or with =1, =ON, or =TRUE. To disable a feature, specify
the suboption with either =0, =OFF, or =FALSE. For example, the
following command lines are equivalent:
f90 -LNO:auto_dist:blocking=OFF:oinvar=FALSE a.f
f90 -LNO:auto_dist=1:blocking=0:oinvar=OFF a.f
For brevity, this man page shows only the ON or OFF settings to
suboptions, but 0, 1, TRUE, and FALSE are also allowed as settings. In
addition, this man page shows the abbreviated form for some of the
suboption names. You can use either the abbreviation or the complete
suboption name when using the suboptions. The following is a list of
the abbreviations and the complete suboption names:
Complete name Abbreviation
outer_unroll ou
associativity assoc
clean_miss_penalty cmp
dirty_miss_penalty dmp
cache_size cs
is_memory_level is_mem
line_size ls
tlb_entries tlb
tlb_clean_miss_penalty tlbcmp
prefetch_level pf
See "F77 LNO Directives" at the end of this man page for a summary of
the F77 directives for LNO. See the MIPSPro 7 Fortran 90 Commands and
Directives Reference Manual, publication SR-3907, for a discussion of
the Fortran 90 LNO directives. See MIPSpro C and C++ Pragmas, for
descriptions of the C and C++ LNO #pragma directives.
The descriptions to the suboptions to -LNO: are divided into the
following categories:
* General options
* Transformation options
* Cache memory management options
* TLB options
* Prefetch options
The -LNO option accepts the following general suboptions:
Suboption Action
auto_dist[ = ( ON|OFF )]
Distributes local arrays in common blocks that are
accessed in parallel. The default is OFF.
This optimization works with either automatic parallelism
or parallelism using directives; it is always safe, and
does not affect the layout of arrays in virtual space, and
does not incur addressing overhead.
fission=n Controls loop fission. n can be one of the following:
0 Disables loop fission.
1 Performs normal fission as necessary. This is the
default.
2 Specifies that fission be tried before fusion.
If -LNO:fission=n and -LNO:fusion=n are both set to 1 or
to 2, fusion is performed.
fusion=n Controls loop fusion. n can be one of the following:
0 Disables loop fusion.
1 Performs standard outer loop fusion. This is the
default.
2 Specifies that outer loops should be fused, even if it
means partial fusion.
The compiler attempts fusion before fission. The compiler
performs partial fusion if not all levels can be fused in
the multiple-level fusion.
If -LNO=fission=n and -LNO:fusion=n are both set to 1 or
to 2, fusion is performed.
fusion_peeling_limit=n
Sets the limit for the number of iterations allowed to be
peeled in fusion, where n >= 0. By default, n=5.
gather_scatter=n
Performs gather-scatter optimizations. n can be one of
the following:
0 Disables all gather-scatter optimization.
1 Performs gather-scatter optimizations on non-nested IF
statements. This is the default.
2 Performs multi-level gather-scatter optimizations.
ignore_pragmas[ = ( ON|OFF )]
Specifies that the command line options override
directives in the source file. The default is OFF.
non_blocking_loads[ = ( ON|OFF )]
(C/C++ and F77 only) Specifies whether the processor
blocks on loads. If not set, the default of the current
processor is used.
oinvar[ = ( ON|OFF )]
Controls outer loop hoisting. The default is ON.
opt=n Controls the LNO optimization level. n can be one of the
following:
0 Disables nearly all loop nest optimization.
1 Peforms full loop nest transformations. This is the
default.
outer[ = ( ON|OFF )]
Enables or disables outer loop fusion. The default is ON.
vintr[ = ( ON|OFF )]
Specifies that vectorizable versions of the math intrinsic
functions should be used. The default is ON.
For information on the math intrinsic functions, see
math(3M).
The loop transformation arguments allow you to control cache blocking,
loop unrolling, and loop interchange. They are as follows:
blocking[ = ( ON|OFF )]
Specify blocking=OFF to disable the cache blocking
transformation. The default is ON.
blocking_size=[n1][,n2]
Specifies a block size that the compiler must use when
performing any blocking. When using the MIPSpro 7 Fortran
90 compiler, specify a value for n2 when using a 2-level
cache. For n1 or n2, specify a positive integer number that
represents the number of iterations.
interchange[ = ( ON|OFF )]
Specifies whether or not loop interchange optimizations are
performed. The default is ON.
ou=n Indicates that all outer loops for which unrolling is legal
should be unrolled by n, where n is a positive integer. The
compiler unrolls loops by this amount or not at all.
ou_deep[ = ( ON|OFF )]
Specifies that for loops with 3-deep, or deeper, loop nests,
the compiler should outer unroll the wind-down loops that
result from outer unrolling loops further out. This results
in large code size, but it generates much faster code
whenever wind-down loop execution costs are important. The
default is ON.
ou_further=n
Specifies whether or not the compiler performs outer loop
unrolling on wind-down loops. Specify an integer for n.
ou_max=n Indicates that the compiler can unroll as many as n copies
per loop, but no more.
ou_prod_max=n
Indicates that the product of unrolling of the various outer
loops in a given loop nest is not to exceed n, where n is a
positive integer. The default is 16.
pwr2[ = ( ON|OFF )]
(C/C++ and F77 only) Specifies whether to ignore the leading
dimension (set to OFF to ignore).
You can disable additional unrolling by specifying
-LNO:ou_further=999999. Unrolling is enabled as much as is
sensible by specifying -LNO:ou_further=3.
Certain arguments allow you to describe the target cache memory
system. The numbering in the following arguments starts with the
cache level closest to the processor and works outward:
assoc1=n, assoc2=n, assoc3=n, assoc4=n
Specifies the cache set associativity. For a fully
associative cache, such as main memory, set n to any
sufficiently large number, such as 128. Specify a positive
integer for n. Specifying n=0 indicates that there is no
cache at that level.
cmp1=n, cmp2=n, cmp3=n, cmp4=n
dmp1=n, dmp2=n, dmp3=n, dmp4=n
Specifies, in processor cycles, the time for a clean miss
(cmpx=) or dirty miss (dmpx=) to the next outer level of the
memory hierarchy. This number is approximate because it
depends upon a clean or dirty line, read or write miss, etc.
Specify a positive integer for n. Specifying n=0 indicates
that there is no cache at that level.
cs1=n, cs2=n, cs3=n, cs4=n
Specifies the cache size. The value n can be 0, or it can
be a positive integer followed by one of the following
letters: k, K, m, or M. This specifies the cache size in
Kbytes or Mbytes. Specifying 0 indicates that there is no
cache at that level.
The default cache size depends on your system. You can use
the -LIST:options=ON option to see the default cache sizes
used during your compilation.
is_mem1[ = ( ON|OFF )]
is_mem2[ = ( ON|OFF )]
is_mem3[ = ( ON|OFF )]
is_mem4[ = ( ON|OFF )]
Specifies that certain memory hierarchies should be modeled
as memory, not cache. The default is OFF for each option.
Blocking can be attempted for this memory hierarchy level,
and blocking appropriate for memory, rather than cache, is
applied. No prefetching is performed, and any prefetching
options are ignored. If an -OPT:is_memx[ = ( ON|OFF )]
option is specified, the corresponding assocx=n
specification is ignored, any cmpx=n and dmpx=n options on
the command line are ignored.
ls1=n, ls2=n, ls3=n, ls4=n
Specifies the line size, in bytes. This is the number of
bytes, specified in the form of a positive integer number,
n, that are moved from the memory hierarchy level further
out to this level on a miss. Specifying n=0 indicates that
there is no cache at that level.
Certain arguments control the TLB. The TLB is a cache for the page
table, and it is assumed to be fully associative. The TLB control
arguments are as follows:
ps1=n, ps2=n, ps3=n, ps4=n
Specifies the number of bytes in a page. Specify a positive
integer for n. The default n depends on your system
hardware.
tlb1=n, tlb2=n, tlb3=n, tlb4=n
Specifies the number of entries in the TLB for this cache
level. Specify a positive integer for n. The default n
depends on your system hardware.
tlbcmp1=n, tlbcmp2=n, tlbcmp3=n, tlbcmp4=n
tlbdmp1=n, tlbdmp2=n, tlbdmp3=n, tlbdmp4=n
Specifies the number of processor cycles it takes to service
a clean TLB miss (the tlbcmpx= options) or dirty TLB miss
(the tlbdmpn= options). Specify a positive integer for n.
The default n depends on your system hardware.
The following arguments control the prefetch operation:
pf1[ = ( ON|OFF )]
pf2[ = ( ON|OFF )]
pf3[ = ( ON|OFF )]
pf4[ = ( ON|OFF )]
Selectively disables and enables prefetching for cache level
x, for pfx[ = ( ON|OFF )]
When -r10000 is in effect, pf1=ON and pf2=ON by default. At
any other -rn setting, OFF is in effect for all cache
levels.
prefetch=n
Specifies levels of prefetching. n can be one of the
following:
0 Disables all prefetching. This is the default when
-r4000, -r5000, or -r8000 is in effect.
1 Enables conservative prefetching. This is the default
when -r10000 is in effect.
2 Enables aggressive prefetching.
prefetch_ahead=n
Prefetches the specified number of cache lines ahead of the
reference. Specify a positive integer for n. The default
is 2.
prefetch_manual[ = ( ON|OFF )]
Specifies whether manual prefetches (through directives)
should be respected or ignored.
prefetch_manual=OFF ignores manual prefetches. This is the
default when -r8000, -r5000, or -r4000 is in effect.
prefetch_manual=ON respects manual prefetches. This is the
default when -r10000 is in effect.
F77 LNO Directives
Directives within a program unit apply only to that program unit,
reverting to the default values at the end of the program unit.
Directives that occur outside of a program unit alter the default
value, and therefore apply to the rest of the file from that point on,
until overridden by a subsequent directive.
Directives within a file override the command line options by default.
To have the command line options override directives, use the command
line option:
-LNO:ignore_pragmas
Fission and Fusion Directives [Toc] [Back]
* C*$* AGGRESSIVE INNER LOOP FISSION: Fission this loop in
inner_fission phase to as many loops as possible. This must be
followed by a inner loop and has no effect if that loop is not inner
any more after the SNL phase.
* C*$* FISSION [(n)] or C*$* FISSIONABLE: Fission the enclosing n
level of loops after this directive. Perform legality test unless a
fissionable directive is also specified. Does not re-order
statements.
* C*$* FUSE [(n [,level] )] or C*$* FUSABLE: Fuse the following n
immediately adjacent loops. Fusion is attempted on each pair of
adjacent loops and the level, by default, is the determined by the
maximal SNL levels of the fused loops, although partial fusion is
allowed. Iterations may be peeled as needed during fusion; the
peeling limit is 5 or the number specified by the
-LNO:fusion_peeling_limit flag. When the FUSABLE directive is
present, no legality test is done and the fusion is done up to the
maximal SNL levels where the iteration numbers matched for each pair
of loops to be matched. The default value for n is 2.
* C*$* NO FISSION: The loop following this directive should not be
fissioned in either fiz_fuse phase or inner_fission phase. Its inner
loops, however, are allowed to be fissioned.
* C*$* NO FUSION: The loop following this directive should not be
fused with other loops.
SNL Transformation Directives [Toc] [Back]
The parallelizing preprocessor may do some transformation for
parallelism that violate some of these directives.
* C*$* INTERCHANGE (I, J [,K ...] ): Loops I, J and K (in any order)
must directly follow this directive and be perfectly nested inside
each other. If they are not perfectly nested, the compiler may
perform loop distribution to make them so, or may ignore the
annotation, or may apply imperfect interchange (this is not likely).
The compiler attempts to reorder loops so that I is outermost, then
J, then K. The compiler may ignore this directive. There must be a
minimum of 2 indexes in the directive.
* C*$* NO INTERCHANGE: Prevents the compiler from involving the loop
directly following this directive in a permutation, or any loop
nested within this loop.
* C*$* BLOCKING SIZE (n1,n2) or C*$* BLOCKING SIZE (n1) or C*$*
BLOCKING SIZE (,n2): If the specified loop is involved in a blocking
for the primary or secondary cache, it will have a blocksize of n1
or n2. The compiler will try to include this loop within such a
block. If a blocking size is specified as 0, the loop is not
actually stripped, but the entire loop is inside the block.
* C*$* NO BLOCKING: Prevent the compiler from involving this loop in a
cache blocking.
* C*$* UNROLL (n [,n2] ): This directive suggests that n-1 copies of
the loop body be added to the inner loop. If the loop that this
directive directly preceeds is an inner loop, then it indicates
standard unrolling. If the loop that this directive directly
preceeds is not innermost, then outer loop unrolling is performed.
n must be at least 1. If n=1 then no unrolling will be performed.
If n=0, then the default unrolling should be applied. n2 is
ignored.
* C*$* BLOCKABLE (I,J [,K ...] ): The I, J and K loops must be
adjacent and nested within each other, although not necessarily
perfectly nested. This directive informs the compiler that these
loops may legally be involved in a blocking with each other, even if
the compiler would consider such a transformation illegal. The
loops are also interchangeable and unrollable. This directive does
not instruct the compiler which of these transformations to apply.
You must specify at least 2 loop indexes in the directive.
Prefetch Directives [Toc] [Back]
* C*$* PREFETCH (n[,n]): Specify prefetching for each level of the
cache. The scope is the entire function containing the directive. n
can be one of the following values:
0 prefetching off (default for all processors except R10000)
1 prefetching on, but conservative
2 prefetching on, and aggressive (default when prefetch is on)
* C*$* PREFETCH_MANUAL (n): Specify if manual prefetches (through
directives) should be respected or ignored. Scope: Entire function
containing the directive. n can be one of the following values:
0 ignore manual prefetches (default for mips3 and earlier)
1 respect manual prefetches (default for mips4)
* C*$* PREFETCH_REF_DISABLE=A [, size=num]: This directive explicitly
disables prefetching all references to array A in the current
function. The auto-prefetcher runs (if enabled) ignoring array A.
The size is used for volume analysis. Scope: Entire function
containing the directive. size=num is the size of the array
references in this loop, in Kbytes. This is an optional argument
and must be a constant.
* C*$* PREFETCH_REF=array-ref,[stride=[str] [,str]], [level=[lev]
[,lev]], [kind=[rd/wr]], [size=[sz]]: This directive generates a
single prefetch instruction to the specified memory location. It
searches for array references that match the supplied reference in
the current loop-nest. If such a reference is found, that reference
is connected to this prefetch node with the specified latency. If no
such reference is found, this prefetch node stays free-floating and
is scheduled "loosely".
All references to this array in this loop-nest are ignored by the
automatic prefetcher (if enabled).
If the size is supplied, then the auto-prefetcher (if enabled)
reduces the effective cache size by that amount in its calculations.
The compiler tries to issue one prefetch per stride iteration, but
cannot guarantee it. Redundant prefetches are preferred to
transformations (such as inserting conditionals) which incur other
overhead.
Scope: No scope. Just generates a prefetch instruction.
The following arguments are used with this option:
array-ref Required. The reference itself, for example, A(i, j).
str Optional. Prefetch every str iterations of this loop. The
default is 1.
lev Optional. The level in memory hierarchy to prefetch. The
default is 2. If lev=1, prefetch from L2 to L1 cache. If
lev=2, prefetch from memory to L1 cache.
rd/wr Optional. The default is read/write.
sz Optional. The size (in Kbytes) of the array referenced in
this loop. This must be a constant.
Dependence Analysis Directives [Toc] [Back]
* CDIR$ IVDEP: This applies only to inner loops. Liberalize dependence
analysis. Given two memory references, where at least one is loop
variant, ignore any loop-carried dependences between the two
references. The following are examples of this directive.
do i = 1,n
b(k) = b(k) + a(i)
enddo
IVDEP does not break the dependence because b(k) is not loop-variant.
do i=1,n
a(i) = a(i-1) + 3
enddo
IVDEP does break the dependence but the compiler warns the user that
it is breaking an obvious dependence.
do i=1,n
a(b(i)) = a(b(i)) + 3.
enddo
IVDEP does break the dependence.
do i = 1,n
a(i) = b(i)
c(i) = a(i) + 3.
enddo
IVDEP does not break the dependence on a[i] because it is within an
iteration.
If -OPT:cray_ivdep=ON, Cray semantics are used and all lexically
backwards dependences are broken. The following are examples:
do i=1,n
a(i) = a(i-1) + 3.
enddo
IVDEP does break the dependence but the compiler warns the user that
it's breaking an obvious dependence.
do i=1,n
a(i) = a(i+1) + 3.
enddo
IVDEP does not break the dependence because the dependence is from the
load to the store, and the load comes lexically before the store.
If -OPT:liberal_ivdep=ON, all dependences are broken.
cc(1), CC(1), cord(1), dso(1), f77(1), f90(1), fpmode(1), hinv(1),
ld(1), make(1), pixie(1), pmake(1), prof(1), rld(1), smake(1).
math(3M).
auto_p(5), gp_overflow(5), ipa(5), opt(5), pe_environ(5).
MIPSpro C and C++ Pragmas, publication 007-3587-001
C Language Reference Manual, publication 007-0701-120
Compiler Information File (CIF) Reference Manual, publication SR-2401
MIPSPro 7 Fortran 90 Commands and Directives Reference Manual,
publication SR-3907
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