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RISC Processors

Chapter 14S. Dandamudi

2003To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.

S. Dandamudi Chapter 14: Page 2

Outline

• Introduction• Evolution of CISC

processors• RISC design principles• PowerPC processor

∗ Architecture∗ Addressing modes∗ Instruction set

• Itanium processor∗ Architecture∗ Addressing modes∗ Instruction set∗ Instruction-level parallelism∗ Branch handling∗ Speculative execution

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Introduction

• CISC∗ Complex instruction set

» Pentium is the most popular example

• RISC∗ Simple instructions

» Reduced complexity

∗ Modern processors use this design philosophy» PowerPC, MIPS, SPARC, Intel Itanium

– Borrow some features from CISC

∗ No precise definition» We can identify some common characteristics



Evolution of CISC Designs

• Motivation to efficiently use expensive resources∗ Processor∗ Memory

• High density code∗ Complex instructions

» Hardware complexity is handled by microprogramming» Microprogramming is also helpful to

– Reduce the impact of memory access latency– Offers flexibility

Low-cost members of the same family

∗ Tailored to high-level language constructs

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Evolution of CISC Designs (cont’d)

CISC RISC

VAX 11/780

Intel 486 MIPS R4000

# instructions 303 235 94

Addr. modes 22 11 1

Inst. size (bytes) 2-57 1-12 4

GP registers 16 8 32



Evolution of CISC Designs (cont’d)

Example∗ Autoincrement addressing mode of VAX

» Performs the following actions:(R2) = (R2) + R3; R2 = R2 + 1

∗ RISC equivalentR4 = (R2)

R4 = R4 + R3

(R2) = R4

R2 = R2 + 1

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Why RISC?

• Simple instructions are preferred∗ Complex instructions are mostly ignored by compilers

» Due to semantic gap• Simple data structures

∗ Complex data structures are used relatively infrequently∗ Better to support a few simple data types efficiently

» Synthesize complex ones• Simple addressing modes

∗ Complex addressing modes lead to variable length instructions

» Lead to inefficient instruction decoding and scheduling



Why RISC? (cont’d)

• Large register set∗ Efficient support for procedure calls and returns

» Patterson and Sequin’s study– Procedure call/return: 12−15% of HLL statements

Constitute 31−33% of machine language instructionsGenerate nearly half (45%) of memory references

∗ Small activation record» Tanenbaum’s study

– Only 1.25% of the calls have more than 6 arguments– More than 93% have less than 6 local scalar variables– Large register set can avoid memory references

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RISC Design Principles

• Simple operations∗ Simple instructions that can execute in one cycle

• Register-to-register operations∗ Only load and store operations access memory∗ Rest of the operations on a register-to-register basis

• Simple addressing modes∗ A few addressing modes (1 or 2)

• Large number of registers∗ Needed to support register-to-register operations∗ Minimize the procedure call and return overhead



RISC Design Principles (cont’d)

Register windows storing activation records

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RISC Design Principles (cont’d)

• Fixed-length instructions∗ Facilitates efficient instruction execution

• Simple instruction format∗ Fixed boundaries for various fields

» opcode, source operands,…

• Other features∗ Tend to use Harvard architecture∗ Pipelining is visible at the architecture level



PowerPC

• Registers∗ 32 general-purpose registers (GPR0 – GPR31)∗ 32 floating-point registers (FPR0 – FPR31)∗ Condition register (CR)

» Similar to Pentium’s flags register» Divided into 8 CR fields (4 bits each)

– “less than” (LT), “greater than” (GT), “equal to” (EQ), Overflow (SO)

– CR1 is for floating-point exceptions– Other CR fields can be used for integer or FP exceptions– Branch instructions can test a specific CR field bit

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PowerPC (cont’d)



PowerPC (cont’d)

∗ XER register serves two distinct purposes» Bits 0, 1, and 2 are used to capture

– Summary overflow (SO), overflow (OV), carry (CA)– OV and CA are similar to Pentium’s overflow and carry– SO, once set, only a special instruction can clear it

» Bits 25 to 31 (7 bits)– Specifies the number of bytes to be transferred between

memory and registers– Two instructions

Load string word indexed (lswx)Store string word indexed (stswx)Can load/store all 32 registers (GPR0-GPR31)

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PowerPC (cont’d)

∗ Link register (LR)» Used to store the procedure return address

– Stores the effective address of the instruction following the procedure call instruction

– Procedure calls use the branch instructionsExample: b = branch, bl = procedure call

∗ Count register (CTR)» Maintains loop count value

– Similar to Pentium's ECX register– Branch instructions can test the value

• 32-bit PowerPC implementations use segmentation like the Pentium



PowerPC (cont’d)

• Addressing modes∗ Load/store instructions support three addressing modes

» Can use GPRs

∗ Register Indirect» Effective address = contents of rA or 0» Specifying 0 generates address 0

∗ Register Indirect with Immediate Index» Effective address = Contents of rA or 0 + imm16

∗ Register Indirect with Index» Effective address = Contents of rA or 0 + contents of rB

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PowerPC (cont’d)

Instruction format



PowerPC (cont’d)

• Bits 0-5∗ Specify primary opcode∗ Other fields specify suboperations

» Depends on instruction type

• AA bit∗ 1 (use absolute address)∗ 0 (use relative address)

• LK bit∗ 0 (no link --- branch)∗ 1 (link --- turns branch into a procedure call)

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PowerPC Instruction Set

• Data Transfer instructions• Byte loads

lbz rD,disp(rA) ;Load byte and zerolbzu rD,disp(rA) ;Load byte and zero

;with update» Effective address = contents of rA + disp

lbzx rD,rA,rB ;Load byte and zero indexedlbzux rD,rA,rB ;Load byte and zero

;with update indexed» Effective address = contents of rA + contents of rB» Upper three bytes of rD are zeroed» Update versions: rA ← effective address



PowerPC Instruction Set (cont’d)

• Similar instructions for halfword and word loadslhz, lhzu, lhzx, lhzxu

lwz, lwzu, lwzx, lwzxu

• For halfword loads, sign extension is possiblelha, lhau, lhax, lhaxu

• Multiword loadlmw rD,disp(rA)

» Loads n consecutive words at EA to registers rD, …, r31

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• Similar instructions for storestbz, stbzu, stbzx, stbzxu

sthz, sthzu, sthzx, sthzxu

stwz, stwzu, stwzx, stwzxu

• Multiword storestmw rD,disp(rA)

» Stores n consecutive words at EA to registers rD, …, r31




Arithmetic Instructions• Add instructions

add rD,rA,rB ; rD ← rA + rB

» Status and overflow bits of CR0 and XER are not altered

add. rD,rA,rB ; alters LT,GT,EQ,SO of CR0addo rD,rA,rB ; alters SO,OV of XERaddo. rD,rA,rB ; alters LT,GT,EQ,SO of CR0

; and SO,OV of XER» These four instructions do not alter the CA bit of XER

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PowerPC Instruction Set (cont’d)∗ To alter CA bit, use

adde rD,rA,rB∗ To alter the other bits, use

adde., addeo, addeo.

∗ Immediate operand version

addi rD,rA,Simm16

∗ We can use addi to implement other instructions

li rD,value as addi rD,0,value

la rD,disp(rA) as addi rD,rA,disp

subi rD,rA,value as addi rD,rA,-value




• Subtract instructions

subf rD,rA,rB ; rD ← rB − rA

–subf = subtract from

∗ Like add, other forms are availablesubf., subfo, subfo.

∗ Negate instructionneg rD,rA ; rD ← 0 − rA

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• Multiply instructions∗ Two instructions to get upper and lower 32 bits of the

64-bit resultmullw rD,rA,rB ; signed/unsigned multiply

» Stores the lower-order 32 bits of the result» Use the following to get the upper 32 bits

mulhw rD,rA,rB ; signedmulhwu rD,rA,rB ; unsigned

∗ Immediate formmulli rD,rA,Simm16

» Stores only lower 32 bits of the 48-bit result




∗ Divide instructions» Two divide instructions

– Signed (divw)

divw rD,rA,rB ; rD = rA/rB– Unsigned (divwu)

» Both give only quotient

» For quotient and remainder, use

divw rD,rA,rB ; quotient in rD

mullw rX,rD,rB

subf rC,rX,rA ; remainder in rC

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∗ Logical instructionsand rD,rS,rB and. rD,rS,rB

andi. rD,rS,Uimm16 andis. rD,rS,Uimm16

andc rD,rS,rB andc. rD,rS,rB

» andis = left shift uimm16 by four positions before ANDing» andc = complement rB before ANDing» Dot versions update the LT, GT, EQ, SO bits of CR0» Logical OR also has these six versions» Move register instruction is implemented using OR

mr rA,RS is equivalent to or rA,rS,rS

» NOP is implemented asori 0,0,0




∗ Other logical operations» NAND

– nand

– nand.

» NOR– nor

– nor.

» XOR– xor, xor.

– xori, xoris

» Equivalence (exclusive-NOR)– eqv

– eqv.

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∗ Shift and Rotate instructions» Shift left

slw rA,rS,rB ; shift left word» Shift left the word in rS by rB positions and store result in rA

– Shifted out bits get zeroes» Also have the dot version slw.» Shift right

srw srw. (logical)sraw sraw. (arithmetic)

» Rotate left instructionsrlwnm rA,rS,rB,MB,ME

rotlw rA,rS,rB ≡≡≡≡ rlwnm rA,rS,rB,0,31




∗ Compare instructions» Two versions:

– For signed and unsigned» Two formats

– Register and immediate» Register compare

cmp crfD,rA,rB» Updates LT (rA < rB), GT (rA > rB), EQ, SO bits in the crfD» If crfD is not specified, CR0 is used» Immediate version

cmp crfD,rA,Simm16

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∗ Branch Instructions» Used for both branch (LK = 0) and procedure calls (LK = 1)» Can use absolute (AA = 1) or relative address (AA = 0)

b target (AA=0, LK=0) Branchba target (AA=1, LK=0) Branch Absolutebl target (AA=0, LK=1) Branch then linkbla target (AA=1, LK=1) Branch Absolute then link

» The last two are procedure calls» Three types of conditional branches

– Direct address– Register indirect

CTR or LR




∗ Conditional branch instructions (direct address)bc BO,BI,target (AA=0, LK=0)

Branch Conditionalbca BO,BI,target (AA=1, LK=0)

Branch Conditional Absolutebcl BO,BI,target (AA=0, LK=1)

Branch Conditional then linkbcla BO,BI,target (AA=1, LK=1)

Branch Conditional Absolute then link» BO = branch options (5 bits) ⇒ specifies branch condition» BI = branch input (5 bits) ⇒ specifies a bit in CR field

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∗ Nine different branch conditions can be specified» Decrement CTR; branch if CTR ≠ 0 AND cond = false

» Decrement CTR; branch if CTR = 0 AND cond = false

» Decrement CTR; branch if CTR ≠ 0 AND cond = true

» Decrement CTR; branch if CTR = 0 AND cond = true

» Branch if cond = false

» Branch if cond = true

» Decrement CTR; branch if CTR ≠ 0

» Decrement CTR; branch if CTR = 0

» Branch always




∗ LR-based branch instructionsbclr BO,BI (LK=0)

Branch Conditional to Link Registerbclrl BO,BI (LK=1)

Branch Conditional to Link Register then Link» Target address is taken from LR» Used to return from procedure calls

∗ CTR-based branch instructionsbcctr BO,BI (LK=0) bcctrl BO,BI (LK=1) » CTR instead of LR is used to get target

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Itanium

• Intel’s 64-bit processor∗ RISC based∗ Based on EPIC design philosophy

» Explicit Parallel Instruction Computing» Support for ILP

– 3-instruction wide word» Speculative computation

– Hides memory latency» Predication

– Improves branch handling» Large number of registers

– 128 integer and 128 FP– Aids in efficient procedure calls



Itanium (cont’d)

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Itanium (cont’d)

• Registers∗ 128 general purpose register (gr0 – gr127)

» 64-bit wide» NaT (Not-a-Thing) bit

– Used in speculative loading» Divided into static and stacked

– StaticFirst 32 registers (gr0 – gr31)gr0 is read-only (always provides zero)

– StackedAvailable for programsUsed as register stack frame



Itanium (cont’d)

• Registers∗ Branch registers

» 8 in total (br0 – br7)» 64-bit wide» Specify target address for

– Conditional branches– Procedure calls– Return

∗ User mask register» Alignment, byte ordering, …

∗ Other registers» Predicate register, Application registers, Current frame marker

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Itanium (cont’d)

• Addressing modes∗ Load/store instructions can access memory

» Specify three registers: r1, r2, r3– r32 and r3 are used to compute effective address– r1 receives/supplies data

∗ Register indirect addressing» Effective address = contents of r3

∗ Register indirect with immediate addressing» Effective address = contents of r3 + imm9» r3 = Effective address

∗ Register indirect with index addressing» Effective address = contents of r3 + contents of r2» r3 = Effective address



Itanium (cont’d)

• Instruction Format[(qp)] mnemonic[.comp] dests = srcs

∗ qp = qualifying predicate» Specifies a predicate register

– 64 1-bit registers– Executed if the specified PR is 1– Otherwise, instruction is treated as NOP

» mnemonic

– Identifies an instruction (e.g., compare)» comp

– Gives more information to completely specify instruction– E.g., Type of comparison is equality

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Itanium (cont’d)



Itanium (cont’d)

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Itanium (cont’d)

• Examplesadd r1 = r2,r3

Predicate instruction(p4) add r1 = r2,r3

add r1 = r2,r3,1

Compare instructionscmp.eq p3 = r2,r4

cmp.gt p2,p3 = r3,r4

Branch instructionbr.cloop.sptk loop_back



Instruction-level Parallelism

• Itanium provides∗ Runtime support for explicit parallelism

– Compiler/assembler can indicate parallelism» Instruction groups

∗ Large number of registers

• Instruction groups∗ Set of instructions that do not have conflicting

dependencies» Can be executed in parallel

∗ Compiler/assembler can indicate this by ;; notation

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Instruction-level Parallelism

• Example: Logical expression with four termsif (r10 || r11 || r12 || r13) {

/* if-block code */

}

can be done using or-tree evaluationor r1 = r10,r11 /* Group 1 */or r2 = r12,r13 ;;

or r3 = r1,r2 /* Group 2 */

Other instructions /* Group 3 */

∗ Processor can execute as many instructions from group as it can

» Depends on the available resources



Itanium Instruction Bundle

• Each instruction is encoded using 41 bits• Three instructions are bundled together

∗ 128-bit Instruction bundle∗ No conflicting dependencies among the three instructions

» Aids in instruction–level parallelism∗ 5-bit template

» Specifies mapping of instruction slots to execution instruction types– Six instruction types

Integer ALU, non-ALU integer, memory, branch, FP, extended

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Itanium Instructions

• Data transfer instructions» Load and store instructions are more complicated than a typical

RISC processor

∗ Load instructions(qp) ldSZ.ldtype.ldhint r1=[r3]

(qp) ldSZ.ldtype.ldhint r1=[r3],r2

(qp) ldSZ.ldtype.ldhint r1=[r3],imm9» Loads SZ bytes from memory

– SZ can be 1, 2, 4, or 8 to load 1, 2, 4, or 8 bytes– Example:

ld8 r5 = [r6]

Locality of memory access

Special load operations:advanced, speculative



Itanium Instructions (cont’d)

• ldtype∗ This completer can be used to specify special load

operations» Advanced

ld8.a r5 = [r6]

» Speculativeld8.s r5 = [r6]

• ldhint∗ Locality of memory access

None – Temporal locality, level 1nt 1 – No temporal locality, level 1nt a – No temporal locality, all levels

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• Store instructions∗ Simpler than load instructions

(qp) stSZ.sttype.sthint r1=[r3]

(qp) stSZ.sttype.sthint r1=[r3],imm9

• Move instructions(qp) mov r1 = r3

(qp) mov r1 = imm2

(qp) mov r1 = imm64

» First two are pseudo-instructions– Implemented using other processor instructions




• Arithmetic instructions∗ Simpler than load instructions

(qp) add r1 = r2,r3

(qp) add r1 = r2,r3,1

(qp) add r1 = imm,r4

∗ Move instruction(qp) mov r1 = r3

implemented as(qp) add r1 = 0,r3

∗ Move instruction(qp) mov r1 = imm22

implemented as(qp) add r1 = imm22,r0

can be imm14or imm22

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• Similar instructions for subtraction• Shift-add

(qp) shladd r1 = r2,count,r3

» Before adding, r2 is left-shifted by count bit positions

• Integer multiply is realized using the xmainstruction and floating-point registers

• No divide instruction∗ Done in software




• Logical instructions∗ AND∗ OR∗ XOR∗ No NOT operation

» Can use and-complement (andcm)– Complements one of the operands before ANDing

• Format(qp) and r1 = r2,r3

(qp) and r1 = imm8,r3

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• Shift instructions∗ Left-shift∗ Right-shift

• Format(qp) shl r1 = r2,r3

(qp) and r1 = imm8,r3

• Right-shift(qp) shr r1 = r2,r3 (signed version)(qp) shr.u r1 = r2,r3 (Unsigned version)




• Compare instructions∗ Format

(qp) cmp.crel.ctype p1,p2 = r2,r3(qp) cmp.crel.ctype p1,p2 = imm8,r3

∗ crel: Type of comparisonCmp type signed unsigned< lt ult

≤≤≤≤ le ule> gt ugt

≥≥≥≥ ge uge= eq eq

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∗ ctype: Specifies how the two predicate registers are to be updated

» Default: – Comparison result in p1 and its complement in p2

» or type– p1 and p2 are set to 1 only if the comparison result is 1– Otherwise, p1 and p2 are not altered– Useful in OR-type simultaneous execution

» andtype– p1 and p2 are set to 0 only if the comparison result is 0– Otherwise, p1 and p2 are not altered– Useful in AND-type simultaneous execution




• Branch instructions∗ Used for jump as well as procedure calls∗ Supports both direct and indirect branching

» All direct branched are IP-relative∗ IP relative form(qp) br.btype.bwh.ph.dh target25

(basic form)(qp) br.btype.bwh.ph.dh b1=target25

(call form)br.btype.bwh.ph.dh target25

(counted loop form)

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∗ Indirect form(qp) br.btype.bwh.ph.dh b2 (basic form)(qp) br.btype.bwh.ph.dh b1=b2 (call form)∗ btype: Type of branch

» cond or none (for basic form)– Branch taken if qp is 1; otherwise not

» To invoke a procedure– Use the call form with btype = call

– Turns branch into a conditional procedure call– Procedure invoked only if qp is 1; otherwise not– Return address is saved in b1 branch register




» Uncounted counted loop version– Set btype = cloop

– Loop count is in application register ar65– If ar65 not zero, decrements and takes branch

» RET version– Use btype = ret

– Should use the indirect form and specify the branch register that has the return address

• Example 1: Conditional skip(p3) br skip or(p3) br.cond skip

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• Example 2: Loop iterates 100 timesmov lc = 100

Loop_back:

. . .

br.cloop loop_back

• Example 3: Procedure call to sum(p0) br.call br2 = sum

• Example 4: Return from a procedure(p0) br.ret br2



Handling Branches

• Three techniques:∗ Branch elimination

» Eliminate branches– Best way to handle branches is not to have branches

Possible to eliminate some types of branches

∗ Branch speedup» Reduce the delay associated with branches

– Reorder instructions– Speculative execution

∗ Branch prediction» Discussed before (see Chapter 8)

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Handling Branches (cont’d)

• Branch elimination in Itanium∗ Can be done using predication

if (R1 == R2)

R3 = R3 + R1;

else

R3 = R3 – R1;

cmp r1,r2je equalsub r3,r1jmp next

equal:add r3,r1

next:

cmp.eq p1,p2 = r1,r2(p1) add r3 = r3,r1(P2) sub r3 = r3,r1



Handling Branches (cont’d)switch (r6){

case 1:

r2 = r3 + r4;

break;

case 2:

r2 = r3 - r4;

break;case 3:

r2 = r3 + r5;

break;

case 4:

r2 = r3 – r5;

break;

}

cmp.eq p1,p0 = r6,1

cmp.eq p2,p0 = r6,2

cmp.eq p3,p0 = r6,3

cmp.eq p4,p0 = r6,4;;

(p1) add r2 = r3,r4

(p2) sub r2 = r3,r4

(p3) add r2 = r3,r5

(p4) sub r2 = r3,r5

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Speculative Execution

• Instructions are executed in expectation that they will be needed∗ Keeps pipeline full∗ Masks memory latency

• Itanium supports two types∗ Handles data dependencies

» Data dependencies are discussed in Chapter 8

∗ Handles control dependencies∗ Both are compiler optimizations

» Reorders instructions



Speculative Execution (cont’d)

Data speculation sub r6 = r7,r8 ;; //cycle 1

sub r9 = r10,r6 //cycle 2ld8 r4 = [r5] ;;

add r11 = r12,r4 ;; //cycle 4

ld8 r4 = [r5] //cycle 1sub r6 = r7,r8 ;;

sub r9 = r10,r6 ;; //cycle 2

add r11 = r12,r4 //cycle 3

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• Ambiguous dependency between first st8 and ld8

sub r6 = r7,r8 ;; //cycle 1

st8 [r9] = r6 //cycle 2ld8 r4 = [r5] ;;

add r11 = r12,r4 ;; //cycle 4

st8 [r10] = r11 //cycle 5




• We can move such load instructions using advance load (ld.a) and check load (ld.c)

ld8.a r4 = [r5] //cycle 0 or earlier. . .

sub r6 = r7,r8 ;; //cycle 1

st8 [r9] = r6 //cycle 2ld8.c r4 = [r5]add r11 = r12,r4 ;;

st8 [r10] = r11 //cycle 3

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• Further improvement with advance check (chk.a)ld8.a r4 = [r5] //cycle -1 or earlier

. . .add r11 = r12,r4 //cycle 1sub r6 = r7,r8 ;;

st8 [r9] = r6 //cycle 2chk.a r4,recover

back:st8 [r10] = r11

recover:ld8 r4 = [r5] // reloadadd r11 = r12,r4 // reexecute addbr back // jump back




• Control speculation∗ To reduce long latency instructions such as loads,

advance them earlier into the code

cmp.eq p1,p0 = r10,10 //cycle 0

(p1) br.cond skip ;; //cycle 0

ld8 r1 = [r2] ;; //cycle 1


skip:

// other instructionsCannot advance because of branch

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ld8.s r1 = [r2] ;; cycle –2 or earlier

//other instructions

cmp.eq p1,p0 = r10,10 //cycle 0

(p1) br.cond skip //cycle 0

chk.s r1,recovery //cycle 0


skip:

//other instructions

recovery:

ld8 r1 = [r2]

br skip

Speculative check chk.sallows us to advance ld8



Branch Prediction

• Branch hints∗ bwh completer (branch whether hint)

spnt static branch not takensptk static branch takendpnt dynamic branch not takendptk static branch not taken

• Prefetch hint (ph)∗ Hint about sequential prefetch

» few or many

• Deallocation hint (dh)∗ Specifies whether branch cache should be cleared

» clr indicates deallocationLast slide