Volume 1, Part 1: Application Programming Model
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There are two categories of software-pipelined loop branch types: counted and while.
Both categories have two forms: top and exit. The “top” variant is used when the loop
decision is located at the bottom of the loop body. A taken branch will continue the loop
while a not-taken branch will exit the loop. The “exit” variant is used when the loop
decision is located somewhere other than the bottom of the loop. A not-taken branch
will continue the loop and a taken branch will exit the loop. The “exit” variant is also
used at intermediate points in an unrolled pipelined loop.
The branch condition of a counted loop branch is determined by the specific counted
loop type (ctop or cexit), the value of the loop count application register (LC), and the
value of the epilog count application register (EC). Note that the counted loop branches
do not use a qualifying predicate. LC is initialized to one less than the number of
iterations for the counted loop and EC is initialized to the number of stages into which
the loop body has been partitioned. While LC is greater than zero, the branch direction
will continue the loop, LC will be decremented, registers will be rotated (rrb’s are
decremented), and PR 16 will be set to 1 after rotation. (For each of the loop-type
branches, PR 63 is written by the branch, and after rotation this value will be in PR 16.)
Execution of a counted loop branch with LC equal to zero signals the start of the epilog.
While in the epilog and while EC is greater than one, the branch direction will continue
the loop, EC will be decremented, registers will be rotated, and PR 16 will be set to 0
after rotation. Execution of a counted loop branch with LC equal to zero and EC equal to
one signals the end of the loop; the branch direction will exit the loop, EC will be
decremented, registers will be rotated, and PR 16 will be set to 0 after rotation. A
counted loop type branch executed with both LC and EC equal to zero will have a
branch direction to exit the loop. LC, EC, and the rrb’s will not be modified (no rotation)
and PR 63 will be set to 0. LC and EC equal to zero can occur in some types of
optimized, unrolled software-pipelined loops if the target of a cexit branch is set to the
next sequential bundle and the loop trip count is not evenly divisible by the unroll
amount.
The direction of a while loop branch is determined by the specific while loop type (wtop
or wexit), the value of the qualifying predicate, and the value of EC. The while loop
branches do not use LC. While the qualifying predicate is one, the branch direction will
continue the loop, registers will be rotated, and PR 16 will be set to 0 after rotation.
While the qualifying predicate is zero and EC is greater than one, the branch direction
will continue the loop, EC will be decremented, registers will be rotated, and PR 16 will
be set to 0 after rotation. The qualifying predicate is one during the kernel and zero
during the epilog. During the prolog, the qualifying predicate may be zero or one
depending upon the scheme used to program the pipelined while loop. Execution of a
while loop branch with qualifying predicate equal to zero and EC equal to one signals
the end of the loop; the branch direction will exit the loop, EC will be decremented,
registers will be rotated, and PR 16 will be set to 0 after rotation. A while loop branch
executed with a zero qualifying predicate and with EC equal to zero has a branch
direction to exit the loop. EC and the rrb’s will not be modified (no rotation) and PR 63
will be set to 0.
For while loops, the initialization of EC depends upon the scheme used to program the
pipelined while loop. Often, the first valid condition for the while loop branch is not
computed until several stages into the prolog. Therefore, software pipelines for while
loops often have several speculative prolog stages. During these stages, the qualifying
predicate can be set to zero or one depending upon the scheme used to program the
loop. If the qualifying predicate is one throughout the prolog, EC will be decremented
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 ...