Branches in a dynamic superscalar

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Branches in a dynamic superscalar

We want to issue a package of dependent inst $^{\rm n}$ s, e.g.

BEQZ R10,Label IF IS

LD $\fbox{F0}$ ,100(R1)
IF IS MEM $\fbox{\sc mem}$   WB

ADDD $\fbox{F2}$ , $\fbox{F0}$ ,F1 IF IS RO RO EX $\fbox{EX}$ WB

LD $\fbox{R2}$ ,100(R1) IF IS MEM $\fbox{\sc mem}$   WB

SUBD F3, $\fbox{F2}$ ,F4 IF IS RO RO RO RO $\fbox{EX}$ EX WB

ADD R4, $\fbox{R2}$ ,R3 IF IS RO RO $\fbox{EX}$ WB

Issue no. 1 2 Suppose branch is last item of prev issue (eg ... we block issue of rest of package) Issue 2 mustn't proceed speculatively Because it will update several registers These registers are used to pass values from inst $^{\rm n}$ to inst $^{\rm n}$ They would have to be reinstated if the branch were mispredicted

l|@ @*13p0.9em BEQZ R10,Label & IF & IS

LD $\fbox{F0}$ ,100(R1) & & IF & IS & MEM& $\fbox{\sc mem}$ &  WB
ADDD $\fbox{F2}$ , $\fbox{F0}$ ,F1 & & IF & IS & RO & RO & EX & $\fbox{EX}$ & WB
LD $\fbox{R2}$ ,100(R1) & & IF & IS & MEM& $\fbox{\sc mem}$ &  WB
SUBD F3, $\fbox{F2}$ ,F4 & & IF & IS & RO & RO & RO & RO & $\fbox{EX}$ & EX & WB
ADD R4, $\fbox{R2}$ ,R3 & & IF & IS & RO & RO & $\fbox{EX}$ & WB
Issue no. & 1 & 2 Suppose branch is last item of prev issue (eg ... we block issue of rest of package) Issue 2 mustn't proceed speculatively Because it will update several registers These registers are used to pass values from inst $^{\rm n}$ to inst $^{\rm n}$ They would have to be reinstated if the branch were mispredicted

6in

BEQZ R10,Label IF IS

LD $\fbox{F0}$ ,100(R1)
IF IS MEM $\fbox{\sc mem}$   WB

ADDD $\fbox{F2}$ , $\fbox{F0}$ ,F1 IF IS RO RO EX $\fbox{EX}$ WB

LD $\fbox{R2}$ ,100(R1) IF IS MEM $\fbox{\sc mem}$   WB

SUBD F3, $\fbox{F2}$ ,F4 IF IS RO RO RO RO $\fbox{EX}$ EX WB

ADD R4, $\fbox{R2}$ ,R3 IF IS RO RO $\fbox{EX}$ WB

Issue no. 1 2 Suppose branch is last item of prev issue (eg ... we block issue of rest of package) Issue 2 mustn't proceed speculatively Because it will update several registers These registers are used to pass values from inst $^{\rm n}$ to inst $^{\rm n}$ They would have to be reinstated if the branch were mispredicted

l|@ @*13p0.9em BEQZ R10,Label & IF & IS

LD $\fbox{F0}$ ,100(R1) & & IF & IS & MEM& $\fbox{\sc mem}$ &  WB
ADDD $\fbox{F2}$ , $\fbox{F0}$ ,F1 & & IF & IS & RO & RO & EX & $\fbox{EX}$ & WB
LD $\fbox{R2}$ ,100(R1) & & IF & IS & MEM& $\fbox{\sc mem}$ &  WB
SUBD F3, $\fbox{F2}$ ,F4 & & IF & IS & RO & RO & RO & RO & $\fbox{EX}$ & EX & WB
ADD R4, $\fbox{R2}$ ,R3 & & IF & IS & RO & RO & $\fbox{EX}$ & WB
Issue no. & 1 & 2
Suppose branch is last item of prev issue (eg ... we block issue of rest of package)
Issue 2 mustn't proceed speculatively
Because it will update several registers
These registers are used to pass values from inst $^{\rm n}$ to inst $^{\rm n}$
They would have to be reinstated if the branch were mispredicted

Next: ... Up: Multiple instructions/cycle Previous: Dynamic superscalar