1:176
Volume 1, Part 2: Predication, Control Flow, and Instruction Stream
By using predication to reduce the number of control flow changes, the fetching
efficiency will generally improve. The only case where predication is likely to reduce
instruction cache efficiency is when there is a large increase in the number of
instructions fetched which are subsequently predicated off. Such a situation uses
instruction cache space for instructions that compute no useful results.
4.3.4.1
Instruction Stream Alignment
For many processors, when a program branches to a new location, instruction fetching
is performed on instruction cache lines. If the target of the branch does not start on a
cache line boundary, then fetching from that target will likely not retrieve an entire
cache line. This problem can be avoided if a programmer aligns instruction groups that
cross more than one bundle so that the instruction groups do not span cache line
boundaries. However, padding all labels would cause an unacceptable increase in code
size. A more practical approach aligns only tops of loops and commonly entered basic
blocks when the first instruction group extends across more than one bundle. That is, if
both of the following conditions are true at some label L, then padding previous
instruction groups so that
L
is aligned on a cache line boundary is recommended:
• The label is commonly branched to from out-of-line. Examples include tops of loops
and commonly executed else clauses.
• The instruction group starting at label
L
extends across more than one bundle.
To illustrate, assume code at label
L
in the segment below is not cache-aligned and that
a cache boundary occurs between the two bundles. If a program were to branch to
L
,
then execution may split issue after the third add instruction even though there are no
resource oversubscriptions or stops:
L:
{ .mii
add
r1=r2,r3
add
r4=r5,r6
add
r7=r8,r9
}
{ .mfb
ld8
r14=[r56] ;;
nop.f
nop.b
}
On the other hand, if
L
were aligned on an even-numbered bundle, then all four
instructions at
L
could issue in one cycle.
4.4
Branch and Prefetch Hints
Branch and prefetch hints are architecturally defined to allow the compiler or hand
coder to provide extra information to the hardware. Compared to hardware, the
compiler has more time, looks at a wider instruction window (including the source), and
performs more analysis. Transfer of this knowledge to the processor can help to reduce
penalties associated with I-cache accesses and branch prediction.
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 248: ...236 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
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 1302: ...402 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
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 ...