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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

NUM0

eINT

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 ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ p

a 6= b ⇔ pa ≥ b ⇔ qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔

pa ≥ b ⇔ qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ p

a ≥ b ⇔ qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔

qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ q

a b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ pa ≥ b ⇔ qa 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

sahf

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

sahf

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 ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ p

a 6= b ⇔ pa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔

pa b ⇔ (p ∨ q) = p ∧ q

Conditional Jumps


a = b ⇔ pa 6= b ⇔ p

a b ⇔ (p ∨ q) = p ∧ q