ARM Cortex-R4 - 5.3 Return stack

Prefetch Unit
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5.3 Return stack
The call-return stack predicts procedural returns that are program flow changes such as loads,
and branch register. The dynamic branch predictor determines if conditional procedure returns
are predicted as taken or not-taken, as described in Branch prediction on page 5-3. The return
stack predicts the target address for unconditional procedure returns, and conditional procedure
returns that have been predicted as taken by the branch predictor.
The return stack consists of a 4-entry circular buffer. When the PFU detects a taken procedure
call instruction, the PFU pushes the return address onto the return stack. The instructions that
the PFU recognizes as procedure calls are, in both the ARM and Thumb instruction sets:
•
BL immediate
•
BLX immediate
•
BLX Rm .
When the return stack detects a taken return instruction, the PFU issues an instruction fetch from
the location at the top of the return stack, and pops the return stack. The instructions that the
PFU recognizes as procedure returns are, in both the ARM and Thumb instruction sets:
•
LDM Rn{!}, {..,pc}
• POP {..,pc}
• LDMIB Rn{!}, {..,pc}
• LDMDA Rn{!}, {..,pc}
• LDMDB Rn{!}, {..,pc}
• LDR pc, [sp], #4
• BX Rm .
Return stack mispredictions can exist when:
• The prediction that a conditional return passed or failed its condition code is not correct.
• The return address of an unconditional or predicted-taken return is not correct.
The return stack has no underflow or overflow detection. Either scenario is likely to cause a
misprediction.
Note
The MOV PC, LR instruction is not decoded and is not predicted as a return.