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Control Structures
CS2253, Owen Kaser
Control Structures
- Implementing familiar HLL control structures:
– if-then – if-then-else – while – do..while
- Omit: switch
- See textbook Chapter 8
Control Structures CS2253, Owen Kaser Control Structures - - PowerPoint PPT Presentation
Control Structures CS2253, Owen Kaser Control Structures - - PowerPoint PPT Presentation
Control Structures CS2253, Owen Kaser Control Structures Implementing familiar HLL control structures: if-then if-then-else while do..while Omit: switch See textbook Chapter 8 Basic Mechanism Essentially, to
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Basic Mechanism
- Essentially, to disrupt the flow of control you need to
set PC (alias R15) to a new value.
- The b command does this
- But so does any other allowable instruction that writes
to R15!
- Consider this instruction:
add R15, R15, R3 shl 2 Number of instructions skipped ahead depends on R3.
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Nesting
- A typical HLL program has nested control structures: if
inside of an if, inside of while...
- We'll look at how to replace a HLL control structure (that
might have another control structure within it) by corresponding assembly language.
- The inner control structure can be replaced similarly.
- In the following templates, the first use of newlabel1 …
newlabel9 means to generate and use a label that was not already in use. Any subsequent occurrence of, say, newlabel1 means to use that same label.
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If Without Else
- Replace if (<condition>) { <body> } by
code to test the condition (often using CMP) b<opposite of condition> newlabel1 code for body newlabel1
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Example
- a1 is in R1, a2 is in R2
- Translate if (a1 >= a2) { a1++;}
cmp R1, R2 blt xyz0001 ; lt is opposite to >= add R1, R1, #1 ; translation of a1++ xyz0001 ; my new label
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ARM Optimization
- If the body doesn't have nested control statements or
- ther statements that set the flags, can have the
following code to test the condition code for the body, with every instruction conditional. Eg cmp R1, R2 addge R1, R1, #1 ; add made conditional on >=
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If With Else
Replace if (<condition>) { <body1>} else {<body2>} with code to test condition b<opposite of condition> newlabel1 code for body1 b newlabel2 newlabel1 code for body2 newlabel2
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Example
- if (a1 >= a2) a1++; else a2++;
- Following the template:
cmp R1, R2 ; a1 >= a2 ?? blt xyz001 add R1, R1, #1 b xyz002 ; don't fall into else code xyz001 add R2, R2, #1 ; the else's body xyz002
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ARM Optimization
- Since the bodies are simple, can use predicated
[i.e., conditional] instructions: cmp R1, R2 addge R1, R1, #1 ; the “then” body addlt R2, R2, #1 ; the “else” body
- Look Ma, no labels and no branching. No
“branch penalty”.
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While Statement
- Recall that a while statement checks the condition before every
iteration, including the first.
- while (<cond>) {<body>} can turn into
b newlabel1 newlabel2 code for <body> newlabel1 code for <cond> b<the condition> newlabel2 Other translations are possible, but this is the book's
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Example
- for (i=0; i<j; i+=2) ++k; ← for is just while disguised.
mov R1, #0 ; say R1 stores I b xyz001 xyz002 add R3, R3, #1 ; body: say R3 has k add R1, R1, #2 ; code for i+=2 xyz001 cmp R1, R2 ; say R2 has j blt xyz002
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Counting Down To Zero
- If you can arrange for your for loops to count
down from N to zero AND if it is guaranteed to do at least one iteration, better to use code like mov R1, #N ; counting down with R1 newlabel1 code for the body of the loop subs R1, R1, #1 ; set the flags bne newlabel1
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Do...While Statement
Translate do { <body> } while (<cond>); as newlabel1 code for <body> code to check condition b<cond> newlabel1
- Slightly simpler than the while loop
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Nesting
- Let's do Euclid's algo together:
while (a != b) if (a>b) a=a-b; else b=b-a;
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Conditional Execution
- Using conditional execution, we can reduce
Euclid's code to GCD CMP R0, R1 SUBGT R0, R0, R1 SUBLT R1, R1, R0 BNE GCD
- Book also shows how to use conditional