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Volume 1, Part 2: Predication, Control Flow, and Instruction Stream
1:165
Almost all Itanium instructions can be tagged with a guarding predicate. If the value
of the guarding predicate is false at execution time, then the predicated instruction’s
architectural updates are suppressed, and the instruction behaves like a nop. If the
predicate is true, then the instruction behaves as if it were unpredicated. There are a
small number of instructions such as unconditional compares and floating-point
square-root and reciprocal approximate instructions whose qualifying predicate do not
operate as described above. See
Part I:, “Application Architecture Guide”
for additional
information.
The following sequence shows a set of predicated instructions:
(p1)
add
r1=r2,r3
(p2)
ld8
r5=[r7]
(p3)
chk.s
r4,recovery
To set the value of a predict register, the architecture provides compare and test
instructions such as those as shown below.
cmp.eq p1,p2=r5,r6
tbit
p3,p4=r6,5
Additionally, a predicate almost always requires a stop to separate its producing
instruction and its use:
cmp.eq p1,p2=r1,r2;;
(p1)
add
r1=r2,r3
The only exception to this rule involves an integer compare or test instruction that sets
a predicate that is used as the condition for a subsequent branch instruction:
cmp.eq p1,p2=r1,r2 // No stop required
(p1)
br.cond some_target
4.2.3
Optimizing Program Performance Using Predication
This section describes predication-related optimizations, their use, and basic
performance analysis techniques. Following are descriptions of optimizations including
if-conversion, misprediction elimination, off-path predication, upward code motion, and
downward code motion.
4.2.3.1
Applying if-Conversion
One of the most important optimizations enabled by predication is the complete
removal of branches from some program sequences. Without predication, the
pseudo-code below would require a branch instruction to conditionally jump around the
if-block code:
if (r4) {
add r1=r2,r3
ld8 r6=[r5]
}
Using predication, the sequence can be written without a branch:
cmp.ne p1,p0=r4,0 ;;// Set predicate reg
(p1)
add
r1=r2,r3
(p1)
ld8 r6=[r5]
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 ...