1:152
Volume 1, Part 2: Memory Reference
3.4.1
Speculation Model in the Intel
®
Itanium
®
Architecture
The limitations imposed by dependencies on instruction scheduling can be solved by
separating the loading of data from the exception handling or the acknowledgment of
data conflicts. The Itanium architecture supports special speculative versions of
instructions to accomplish this:
• Control speculative load instructions defer exceptions.
• Data speculative load instructions save address information.
• Special check instructions check for exceptions or data conflicts.
An Itanium speculative load can be moved above a dependency barrier (shown as a
dashed line) as shown in
The check detects a deferred exception or a conflict with an intervening store and
provides a mechanism to recover from failed speculation. With this support, speculative
loads and their uses can be scheduled earlier than non-speculative instructions. As a
result, the memory latencies of these loads can be hidden more easily than for
non-speculative loads.
3.4.2
Using Data Speculation in the Intel
®
Itanium
®
Architecture
Data speculation in the Itanium architecture uses a special load instruction (
ld.a
)
called an
advanced load
instruction and an associated check instruction (
chk.a
or
ld.c
)
to validate data-speculated results.
When the
ld.a
instruction is executed, an entry is allocated in a hardware structure
called the Advanced Load Address Table (ALAT). The ALAT is indexed by physical
register number and records the load address, the type of the load, and the size of the
load.
A check instruction must be executed before the result of an advanced load can be used
by any non-speculative instruction. The check instruction must specify the same
register number as the corresponding advanced load.
When a check instruction is executed, the ALAT is searched for an entry with the same
target physical register number and type. If an entry is found, execution continues
normally with the next instruction.
Figure 3-2.
Speculation Model in the Intel
®
Itanium
®
Architecture
Control or
Data Dependency
Original Load
Uses of Load
Speculative Load
Control or
Data Dependency
Check for Exception or
Uses of Load
Memory Conflict
Before Speculation
After Speculation
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 ...