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TriCore 32-bit Unified Processor
Compiler Writer’s Guide
conditional, the actual branch direction is resolved. If the branch direction is contrary to
the prediction, then the correct target address is sent to the fetch unit at the end of the
execute cycle.
The decode cycle for the first instruction at the branch target address follows the execute
cycle for the branch instruction. This is equivalent to a branch time of two cycles for a
branch that is either unconditional, or correctly predicted as taken. If a branch is
incorrectly predicted, the correct target address is sent to the instruction fetch unit at the
end of the branch instruction's execute cycle, giving an effective branch time of three
cycles. For a branch that is correctly predicted as not taken, the execution time is the
same as for a regular instruction: one cycle.
The branch timing numbers are summarized in Table 6, which follows.
Table 6
TriCore 1.2/1.3 Branch Timings
Actual Direction
Predicted Direction
Taken
Not Taken
Taken
2
3
Not Taken
3
1
2.1.3.3
TriCore 2.0 Branch Timings
The TriCore 2.0 branch mechanism is compliant with TriCore 1.2/1.3, but is implemented
differently. Branch instruction selection and hence block ordering should be performed
in the same manner as for TriCore 1.2/1.3 to limit the differences in compiler
implementations.
2.1.3.4
Block Ordering for TriCore 1.2/1.3
Compilers often have some knowledge of branch probabilities. That knowledge may be
derived from heuristics applied to the source code, or from direct feedback of profiling
information. In either case the compiler can use the information to reduce branch costs.
The conditional branch instructions of the TriCore architecture do not include a static
prediction bit as such, but the compiler can choose to order code blocks in such a way
that the static prediction scheme employed by the implementation becomes “self-full
filling”. That scheme is:
• Backward and short forward branches taken
• Long forward branches not taken
A short forward branch is a 16-bit branch instruction with a positive displacement.
Although it is common to order blocks for code generation according to their natural
order in the source code, there is no inherent requirement for the compiler to do so.
Ordering according to the source code does facilitate debugging, and it normally gives
User’s Manual
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V1.4, 2003-12