Volume 1, Part 2: Predication, Control Flow, and Instruction Stream
1:173
Initialization code must be placed in an instruction group prior to the parallel compare.
However, since the initialization code has no dependencies on prior values, it can
generally be scheduled without contributing to the critical path of the code.
The instructions below shows how to generate code for the example above using
parallel compares:
cmp.ne
p1,p0 = r0,r0;; // initialize p1 to 0
cmp.ne.or
p1,p0 = rA,r0
cmp.ne.or
p1,p0 = rB,r0
cmp.ne.or
p1,p0 = rC,r0
cmp.ne.or
p1,p0 = rD,r0
(p1)
br.cond if_block
It is also possible to use
p1
to predicate the if-block in-line to avoid a possible
misprediction. More complex conditional expressions can also be generated with
parallel compares:
if ((rA < 0) && (rB == -15) && (rC > 0))
/* If-block instructions */
The assembly pseudo-code below shows a possible sequence for the C code above:
cmp.eq
p1,p0=r0,r0;; // initialize p1 to 1
cmp.ne.and p1,p0=rB,-15
cmp.ge.and p1,p0=rA,r0
cmp.le.and p1,p0=rC,r0
When used correctly,
and
or
compares write both target predicates with the same value
or do not write the target predicate at all. Another variation on parallel compare usage
is where both the if and else part of a complex conditional are needed:
if ( rA == 0 || rB == 10 )
r1 = r2 + r3;
else
r4 = r5 - r6;
Parallel compares have an
andcm
variant that computes both the predicate and its
complement simultaneously.
cmp.ne
p1,p2 = r0,r0;; // initialize p1,p2
cmp.eq.or.andcmp1,p2 = rA,r0
cmp.eq.or.andcmp1,p2 = rB,10;;
(p1)
add
r1=r2,r3
(p2)
sub
r4=r5,r6
Clearly, these instructions can be used in other combinations to create more complex
conditions.
4.3.2
Reducing Critical Path with Multiway Branches
While there are no special instructions to support branches with multiple conditions and
multiple targets, the Itanium architecture has implicit support by allowing multiple
consecutive B-slot instructions within an instruction group.
Содержание ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3
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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 ...
Страница 13: ...1 2 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 33: ...1 22 Volume 1 Part 1 Introduction to the Intel Itanium Architecture ...
Страница 57: ...1 46 Volume 1 Part 1 Execution Environment ...
Страница 147: ...1 136 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 149: ...1 138 Volume 1 Part 2 About the Optimization Guide ...
Страница 191: ...1 180 Volume 1 Part 2 Predication Control Flow and Instruction Stream ...
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Страница 249: ...2 1 Intel Itanium Architecture Software Developer s Manual Rev 2 3 Part I System Architecture Guide ...
Страница 250: ...2 2 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
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 ...
Страница 750: ...2 502 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
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 ...
Страница 862: ...2 614 Volume 2 Part 2 External Interrupt Architecture ...
Страница 870: ...2 622 Volume 2 Part 2 Performance Monitoring Support ...
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Страница 941: ...3 42 Volume 3 Instruction Reference cmp illegal_operation_fault PR p1 0 PR p2 0 Interruptions Illegal Operation fault ...
Страница 1099: ...3 200 Volume 3 Instruction Reference padd Interruptions Illegal Operation fault ...
Страница 1191: ...3 292 Volume 3 Pseudo Code Functions Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Страница 1295: ...3 396 Volume 3 Resource and Dependency Semantics ...
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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 ...
Страница 1568: ...4 266 Volume 4 Base IA 32 Instruction Reference LGS Load Full Pointer See entry for LDS LES LFS LGS LSS ...
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 ...