Volume 1, Part 2: Software Pipelining and Loop Support
1:187
and a decision is made to exit the loop. The special case in which a software-pipelined
loop branch is executed with
EC
equal to 0 can occur in unrolled software-pipelined
loops if the target of the
cexit
branch is set to the next sequential bundle.
There are two types of software-pipelined loop branches for counted loops.
br.ctop
is
taken when a decision to continue kernel loop execution is made, and is not taken
otherwise. It is used when the loop execution decision is located at the bottom of the
loop.
br.cexit
is not taken when a decision to continue kernel loop execution is made,
and is taken otherwise. It is used when the loop execution decision is located
somewhere other than the bottom of the loop.
5.4.3.2
Counted Loop Example
A conceptual view of a pipelined iteration of the example counted loop on
with II equal to one is shown below:
stage 1:(p16)
ld4 r4 = [r5],4
stage 2:(p17)
---
// empty stage
stage 3:(p18)
add r7 = r4,r9
stage 4:(p19)
st4 [r6] = r7,4
To generate an efficient pipeline, the compiler must take into account the latencies of
instructions and the available functional units. For this example, the load latency is two
and the load and add are scheduled two cycles apart. The pipeline below is coded
assuming there are two memory ports and the loop count is 200.
Figure 5-1.
ctop and cexit Execution Flow
000915
EC?
LC?
LC - -
LC = LC
LC = LC
LC = LC
EC = EC
EC - -
EC - -
EC = EC
PR[63] = 0
PR[63] = 0
PR[63] = 0
PR[63] = 1
RRB - -
RRB - -
RRB - -
RRB = RRB
ctop, cexit
== 0 (epilog)
! = 0
> 1
== 0
==1
(prolog / kernel)
(special unrolled loops)
ctop: branch
cexit: fall-thru
ctop: fall-thru
cexit: branch
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 ...