1:154
Volume 1, Part 2: Memory Reference
The compiler could move up not only the load, but also one or more of its uses. This
transformation uses a
chk.a
rather than a
ld.c
instruction to validate the advanced
load. Using the same example code sequence but now advancing the
add
as well as the
ld8
results in:
ld8.a
r6=[r8];;
// Cycle -3
// other instructions
add
r5=r6,r7
// Cycle -1: add that uses r6
// Other instructions
st8
[r4]=r12
// Cycle 0
chk.a
r6,recover // Cycle 0: check
back: // Return point from jump to recover
st8
[r18]=r5
// Cycle 0
Recovery code must also be generated:
recover:
ld8
r6=[r8] ;; // Reload r6 from [r8]
add
r5=r6,r7
// Re-execute the add
br
back
// Jump back to main code
If the speculation fails, the check instruction branches to the label
recover
where the
speculated code is re-executed. If the speculation succeeds, execution time of the
transformed code is three cycles less than the original code.
3.4.2.3
Terminology Review
Terms related to speculation, such as
advanced
loads
and
check
loads
, have
well-defined meanings in the Itanium architecture. The terms below were introduced in
the preceding sections:
• Data speculative load
A speculative load that is statically scheduled prior to one or more stores upon
which it may be dependent. The data speculative load instruction is
ld.a
.
• Advanced load
A data speculative load.
• Check load
An instruction that checks whether a corresponding advanced load needs to be
re-executed and does so if required. The check load instruction is
ld.c
.
• Advanced load check
An instruction that takes a register number and an offset to a set of
compiler-generated instructions to re-execute speculated instructions when
necessary. The advanced load check instruction is
chk.a
.
• Recovery code
Program code that is branched to by a speculation check. Recovery code repeats a
load and chain of dependent instructions to recover from a speculation failure.
Summary of Contents for ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3
Page 1: ......
Page 11: ...x Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 13: ...1 2 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 33: ...1 22 Volume 1 Part 1 Introduction to the Intel Itanium Architecture ...
Page 57: ...1 46 Volume 1 Part 1 Execution Environment ...
Page 147: ...1 136 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 149: ...1 138 Volume 1 Part 2 About the Optimization Guide ...
Page 191: ...1 180 Volume 1 Part 2 Predication Control Flow and Instruction Stream ...
Page 230: ......
Page 248: ...236 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 250: ...2 2 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 264: ...2 16 Volume 2 Part 1 Intel Itanium System Environment ...
Page 380: ...2 132 Volume 2 Part 1 Interruptions ...
Page 398: ...2 150 Volume 2 Part 1 Register Stack Engine ...
Page 486: ...2 238 Volume 2 Part 1 IA 32 Interruption Vector Descriptions ...
Page 750: ...2 502 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 754: ...2 506 Volume 2 Part 2 About the System Programmer s Guide ...
Page 796: ...2 548 Volume 2 Part 2 Interruptions and Serialization ...
Page 808: ...2 560 Volume 2 Part 2 Context Management ...
Page 842: ...2 594 Volume 2 Part 2 Floating point System Software ...
Page 850: ...2 602 Volume 2 Part 2 IA 32 Application Support ...
Page 862: ...2 614 Volume 2 Part 2 External Interrupt Architecture ...
Page 870: ...2 622 Volume 2 Part 2 Performance Monitoring Support ...
Page 891: ......
Page 1099: ...3 200 Volume 3 Instruction Reference padd Interruptions Illegal Operation fault ...
Page 1295: ...3 396 Volume 3 Resource and Dependency Semantics ...
Page 1296: ......
Page 1302: ...402 Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 1494: ...4 192 Volume 4 Base IA 32 Instruction Reference FWAIT Wait See entry for WAIT ...
Page 1647: ...Volume 4 Base IA 32 Instruction Reference 4 345 ROL ROR Rotate See entry for RCL RCR ROL ROR ...
Page 1884: ...4 582 Volume 4 IA 32 SSE Instruction Reference ...
Page 1885: ...Index Intel Itanium Architecture Software Developer s Manual Rev 2 3 Index ...
Page 1886: ...Index Intel Itanium Architecture Software Developer s Manual Rev 2 3 ...
Page 1898: ...INDEX Index 12 Index for Volumes 1 2 3 and 4 ...