IBM 750GL, 750GX 6.4.1.2 Branch Instructions and Completion

User’s Manual

IBM PowerPC 750GX and 750GL RISC Microprocessor

Figure 6-7. Branch Taken

		Branch Folding				Branch Folding
	(Taken Branch/BTIC Hit)					(Taken Branch/BTIC Miss)
	Clock 0		Clock 1	Clock 2		Clock 0	Clock 1	Clock 2
IQ5	add5				IQ5	add5
IQ4	add4				IQ4	add4
IQ3	add3			and6	IQ3	add3		and4
IQ2	b			and5	IQ2	b		and3
IQ1	add2		and2	and4	IQ1	add2		and2
IQ0	add1		and1	and3	IQ0	add1		and1

Figure 6-8shows the removal of fall-through branch instructions, which occurs when a branch is not taken or is predicted as not taken.

Figure 6-8. Removal of Fall-Through Branch Instruction

Branch Fall-Through

(Not-Taken Branch)

	Clock 0	Clock 1	Clock 2
IQ5	add5	add8	etc.
IQ4	add4	add7	add9
IQ3	add3	add6	add8
IQ2	b	add5	add7
IQ1	add2	add4	add6
IQ0	add1	add3	add5

When a branch instruction is detected before it reaches a dispatch position, and if the branch is correctly predicted as taken, folding the branch instruction (and any instructions from the incorrect path) reduces the latency required for flow control to zero. Instruction execution proceeds as though the branch was never there.

The advantage of removing the fall-through branch instructions at dispatch is only marginally less than that of branch folding. Because the branch is not taken, only the branch instruction needs to be discarded. The only cost of expelling the branch instruction from one of the dispatch entries rather than folding it is missing a chance to dispatch an executable instruction from that position.

6.4.1.2 Branch Instructions and Completion

As described in the previous section, instructions that do not update either the LR or CTR are removed from the instruction stream before they reach the completion queue, either for branch taken or by removing fall- through branch instructions at dispatch. However, branch instructions that update the architected LR and CTR must do so in program order. Therefore, they must perform write-back in the completion stage, like the instructions that update the FPRs and GPRs.

Branch instructions that update the CTR or LR pass through the instruction queue like nonbranch instruc- tions. At the point of dispatch, however, they are not sent to an execution unit, but rather are assigned a slot in the completion queue, as shown in Figure 6-9on page 228.

gx_06.fm.(1.2)	Instruction Timing
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