decision structures - code generationpeople.hsc.edu/faculty-staff/robbk/coms480/lectures... ·...

Decision Structures - Code GenerationLecture 30

Intel Manual, Vol. 1, Sections 3.4.3, 8.1.4Intel Manual, Vol. 2, Chapter 3, jcc

Robb T. Koether

Hampden-Sydney College

Mon, Apr 6, 2015

Robb T. Koether (Hampden-Sydney College) Decision Structures - Code Generation Mon, Apr 6, 2015 1 / 32

1 Conditional Branches

2 The CMPNE TreeInteger ComparisonsFloating-Point Comparisons

3 Jump Trees

4 Assignment

Outline

3 Jump Trees

4 Assignment

Conditional Branches

Branching on a conditional expression will be done in two parts.Evaluate the expression, leaving either true (1) or false (0) on thestack.Test the value on the stack and branch on true.

It would be more efficient to test the original expression andbranch immediately.However, by separating the two tests, it will be simpler to write thecode.

Conditional Branches

Recall that the if statement is of the formif ( expr ) stmt

where expr is either zero (false) or nonzero (true).Thus, we must compare expr to 0.First, we generate code for a CMPNE tree that will leave 1 on thestack if expr is nonzero and 0 on the stack if expr is zero.

Outline

3 Jump Trees

4 Assignment

The CMPNE Tree

CMPNEINT

CMPNEDOUBLE

eDOUBLE

NUM0.0

Integer Expression Floating-point Expression

Outline

3 Jump Trees

4 Assignment

Integer Comparisons

To compare two integers, we mustPop them off the stack into registers.Compare the registers.Push 0 or 1 onto the stack.

There is only one compare instruction: cmp.It sets various flags, depending on the result of the comparison.

Flags CF, ZF, SF, and OF

The relevant flags areCF - carry flagZF - zero flagSF - sign flagOF - overflow flag

Conditional Jumps

We then do a conditional jump.A conditional jump checks the flags to decide whether or not tojump.

Mnemonic Meaning Flagsje jump == ZF = 1

jne jump != ZF = 0jl jump < SF 6= OF

jge jump >= SF = OFjg jump > ZF = 0 and SF = OFjle jump <= ZF = 1 or SF 6= OF

Conditional Jumps

Verify the previous table.Let a and b be numbers.Let p be the statement “ZF = 1” and let q be the statement “SF =OF.”Then

a = b ⇔

pa 6= b ⇔ pa ≥ b ⇔ qa < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ p

a 6= b ⇔ pa ≥ b ⇔ qa < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔

pa ≥ b ⇔ qa < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ p

a ≥ b ⇔ qa < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔

qa < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ q

a < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa < b ⇔

qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa < b ⇔ q

a ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa < b ⇔ qa ≤ b ⇔

p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa < b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a > b ⇔

(p ∨ q) = p ∧ q

Conditional Jumps

a > b ⇔ (p ∨ q) = p ∧ q

Integer Comparison

Integer Comparison for !=mov $1,%ecx # Move true to ecxpop %eax # Pop right operandpop %edx # Pop left operandcmp %eax,%edx # Compare operandsjne L0n # Jump if left != rightdec %ecx # Change ecx to false

L0n:push %ecx # Push T/F result

Outline

3 Jump Trees

4 Assignment

Floating-Point Comparisons

When we compare two floating-point numbers, they are alreadyon the FPU stack, in st(0) and st(1).On older x87 FPUs (earlier than P6), we use the instructionfucompp

Compares st(0) to st(1).Sets condition codes C0 and C3 in the FPU status word.Pops both operands off the stack.

Flags C0 and C3

The result of the comparison is determined by C0 and C3.

Result C0 C3st(0) > st(1) 0 0st(0) < st(1) 1 0st(0) = st(1) 0 1

To perform a conditional jump based on a floating-pointcomparison, we must first move the FPU status word to theEFLAGS register.The instruction

fnstsw %ax

stores the FPU status word in ax (16 bits).The instruction

stores ah (8 bit) in the EFLAGS register.

On newer x87 FPRs (P6 or later), we use the instruction fcomip.Compares st(0) to st(1).Sets flags ZF, SF, and OF in the x86 status word.Pops one operand off the stack.

Flags C0 and C3

The result of the comparison is determined by C0 and C3.

Result C0 C3st(0) > st(1) 0 0st(0) < st(1) 1 0st(0) = st(1) 0 1

To perform a conditional jump based on a floating-pointcomparison, we must first move the FPU status word to theEFLAGS register.The instruction

fnstsw %ax

stores the FPU status word in ax (16 bits).The instruction

stores ah (8 bit) in the EFLAGS register.

The sahf instruction mapsC0→ CF.C3→ ZF.

Thus, we want to use conditional jumps that check CF and ZF.

Conditional Jumps

Mnemonic Meaning Flagsje jump == ZF = 1

jne jump != ZF = 0jb jump < CF = 1

jae jump >= CF = 0ja jump > ZF = 0 and CF = 0

jbe jump <= ZF = 1 or CF = 1

Conditional Jumps

Verify the previous table.Let a and b be numbers.Let p be the statement “ZF = 1” and let q be the statement “CF =1.”Then

a = b ⇔

pa 6= b ⇔ pa < b ⇔ qa ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ p

a 6= b ⇔ pa < b ⇔ qa ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔

pa < b ⇔ qa ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ p

a < b ⇔ qa ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa < b ⇔

qa ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa < b ⇔ q

a ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa < b ⇔ qa ≥ b ⇔

qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa < b ⇔ qa ≥ b ⇔ q

a ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa < b ⇔ qa ≥ b ⇔ qa ≤ b ⇔

p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a = b ⇔ pa 6= b ⇔ pa < b ⇔ qa ≥ b ⇔ qa ≤ b ⇔ p ∨ q

a > b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps

a > b ⇔

(p ∨ q) = p ∧ q

Conditional Jumps

a > b ⇔ (p ∨ q) = p ∧ q

Floating-Point Comparison for !=mov $1,%ecx # Move true to ecxfucompp # Compare and pop operandsfnstsw %ax # Store status wordsahf # Move ah to eflagsjne L0n # Jump if lft != rgtdec %ecx # Change ecx to false

Earlier than P6

Floating-Point Comparison for !=mov $1,%ecx # Move true to ecxfcomip # Compare and pop left operandfstpl -8(%esp) # Pop FPU stackjne L0n # Jump if lft != rgtdec %ecx # Change ecx to false

P6 or later

One little complication is that fucompp and fcomip comparest(0) and st(1) in the reverse order from what we wouldexpect.Therefore, the meanings of < and > are reversed and themeanings of <= and >= are reversed.So we reverse the roles of ja and jb, and jae and jbe.

ARRGGGHHHHH!!!

One little complication is that fucompp and fcomip comparest(0) and st(1) in the reverse order from what we wouldexpect.Therefore, the meanings of < and > are reversed and themeanings of <= and >= are reversed.So we reverse the roles of ja and jb, and jae and jbe.ARRGGGHHHHH!!!

Outline

3 Jump Trees

4 Assignment

Jump Trees

BLABEL6

CMPNEe.mode

JUMP Syntax Tree

JUMPT Syntax Tree

ee.mode

JUMP Trees

A JUMP tree generates the following code.

Code for JUMPjmp B6

JUMPT Trees

A JUMPT tree must pop a boolean value off the stack and jumponly if it is 1.

Code for JUMPTpop %eax # Pop boolean valuecmp $1,%eax # Compare to trueje B6 # Jump if true

Outline

3 Jump Trees

4 Assignment

Assignment

HomeworkRead the Intel Manual, Vol. 1, Sections 3.4.3, 8.1.4.Read about the instructions fcomip (and fucompp, fnstsw, andsahf).Read the Intel Manual, Vol. 2, jcc instructions.

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