1:166
Volume 1, Part 2: Predication, Control Flow, and Instruction Stream
The process of predicating instructions in conditional blocks and removing branches is
referred to as
if-conversion
. Once if-conversion has been performed, instructions can
be scheduled more freely because there are fewer branches to limit code motion, and
there are fewer branches competing for issue slots.
In addition to removing branches, this transformation will make dynamic instruction
fetching more efficient since there are fewer possibilities for control flow changes.
Under more complex circumstances, several branches can be removed. The following C
code sequence:
if (r1)
r2 = r3 + r4;
else
r7 = r6 - r5;
can be rewritten in Itanium architecture-based assembly code without branches as:
cmp.ne p1,p2 = r1,0;;
(p1)
add
r2 = r3,r4
(p2)
sub
r7 = r6,r5
Since instructions from opposite sides of the conditional are predicated with
complementary predicates they are guaranteed not to conflict, hence the compiler has
more freedom when scheduling to make the best use of hardware resources. The
compiler could also try to schedule these statements with earlier or later code since
several branches and labels have been removed as part of if-conversion.
Since the branches have been removed, no branch misprediction is possible and there
will be no pipeline bubbles due to taken branches. Such effects are significant in many
large applications, and these transformations can greatly reduce branch-induced stalls
or flushes in the pipeline.
Thus, comparing the cost of the code above with the non-predicated version above
shows that:
• Non-predicated code consumes: 2 (30% * 10 cycles) = 5 cycles.
• Predicated code consumes: 2 cycles.
In this case, predication saves an average of three cycles.
4.2.3.2
Off-path Predication
If a compiler has dynamic profile information, it is possible to form an instruction
schedule based on the control flow path that is most likely to execute – this path is
called the main trace. In some cases, execution paths not on the main trace are still
executed frequently, and thus it may be beneficial to use predication to minimize their
critical paths as well.
The main trace of a flow graph is highlighted in
. Although blocks A and B are
not on the main trace, suppose they are executed a significant number of times.
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Страница 11: ...x Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 12: ...1 1 Intel Itanium Architecture Software Developer s Manual Rev 2 3 Part I Application Architecture Guide ...
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Страница 33: ...1 22 Volume 1 Part 1 Introduction to the Intel Itanium Architecture ...
Страница 57: ...1 46 Volume 1 Part 1 Execution Environment ...
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Страница 191: ...1 180 Volume 1 Part 2 Predication Control Flow and Instruction Stream ...
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Страница 264: ...2 16 Volume 2 Part 1 Intel Itanium System Environment ...
Страница 380: ...2 132 Volume 2 Part 1 Interruptions ...
Страница 398: ...2 150 Volume 2 Part 1 Register Stack Engine ...
Страница 486: ...2 238 Volume 2 Part 1 IA 32 Interruption Vector Descriptions ...
Страница 749: ...2 501 Intel Itanium Architecture Software Developer s Manual Rev 2 3 Part II System Programmer s Guide ...
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Страница 754: ...2 506 Volume 2 Part 2 About the System Programmer s Guide ...
Страница 796: ...2 548 Volume 2 Part 2 Interruptions and Serialization ...
Страница 808: ...2 560 Volume 2 Part 2 Context Management ...
Страница 842: ...2 594 Volume 2 Part 2 Floating point System Software ...
Страница 850: ...2 602 Volume 2 Part 2 IA 32 Application Support ...
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Страница 1191: ...3 292 Volume 3 Pseudo Code Functions Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
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Страница 1494: ...4 192 Volume 4 Base IA 32 Instruction Reference FWAIT Wait See entry for WAIT ...
Страница 1564: ...4 262 Volume 4 Base IA 32 Instruction Reference LES Load Full Pointer See entry for LDS LES LFS LGS LSS ...
Страница 1565: ...Volume 4 Base IA 32 Instruction Reference 4 263 LFS Load Full Pointer See entry for LDS LES LFS LGS LSS ...
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Страница 1583: ...Volume 4 Base IA 32 Instruction Reference 4 281 LSS Load Full Pointer See entry for LDS LES LFS LGS LSS ...
Страница 1647: ...Volume 4 Base IA 32 Instruction Reference 4 345 ROL ROR Rotate See entry for RCL RCR ROL ROR ...
Страница 1663: ...Volume 4 Base IA 32 Instruction Reference 4 361 SHL SHR Shift Instructions See entry for SAL SAR SHL SHR ...
Страница 1668: ...4 366 Volume 4 Base IA 32 Instruction Reference SIDT Store Interrupt Descriptor Table Register See entry for SGDT SIDT ...
Страница 1884: ...4 582 Volume 4 IA 32 SSE Instruction Reference ...
Страница 1885: ...Index Intel Itanium Architecture Software Developer s Manual Rev 2 3 Index ...
Страница 1886: ...Index Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 1898: ...INDEX Index 12 Index for Volumes 1 2 3 and 4 ...