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If-statement with a compound condition with an OR clause

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• Short-circuit evaluation for an OR clause
  • Consider this if-statement with an or logical operator:

    if ( cond or "Other conditions" ) statement1; else statement2;

    The compound condition is:

    cond or "Other conditions"

    When we evaluation the condition cond at the left hand side, there are 2 possible outcomes:

    cond ≡ true: In this case, we know the outcome of cond or "Other conditions" ≡ true !!! (Because true or any-boolean-value ≡ true) cond ≡ false: In this case, we must evaluate the "Other conditions" to determine the outcome of cond or "Other conditions"

    So in the case that cond ≡ true, we can take the then branch of the program flow immediately (and terminate the boolean evaluation prematurely !!!)

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  • We can use this short-circuit boolean evalation to draw up the following program flow control where we make the branch decision immediately after evaluating a part of the boolean expression:

    if ( cond or "Other conditions" ) ----> | statement1; V else +--------------+ TRUE statement2; | eval cond |--------+ +--------------+ | | | | FALSE | V | +--------------+ FALSE | | eval theRest |--------|---+ +--------------+ | | | TRUE | | | | | V | | statement1 <---------+ | | | +---------+ | | | | +--------------------+ | | | V | statement2 | | | | +-------->+ | | V

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  • The assembler construct that realizes the program control flow of the if-statement with an compound condition "cond or OtherConditions" is as follows:

    Evaluate "cond" (with cmp) TRUE Branch on TRUE outcome of "cond" to label A: ---------------+ | | | FALSE | V | Evaluate "Other Conditions" (another cmp) | | FALSE Branch on FALSE outcome of "Other Conditions" to label B: --|-----+ | | | | TRUE | | V | | A: "statement1" assembler code <--------------------------+ | | | Branch always to label C: ---------------+ | | | | | B: "statement2" assembler code <---------|----------------------+ | | V | C: +<-------------------------------+ | V

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• If-statement with an OR clause - assembler example

  • High level programming langange:

    int x, y, a; if (x <= a || x >= b) x = x + 1; else x = x - 1;

    The flow chart of the above program is:

    The assembler program code for this compound if-statement is:

    /* -------------------------------------------------- <----a b-----> ------+----------+-------- if (x <= a or x >= b) x = x + 1; else x = x - 1; -------------------------------------------------- */ .text main: // Test if ( x <= a ) movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r0, [r0] // r0 = x movw r1, #:lower16:a movt r1, #:upper16:a // r1 = addr(a) ldr r1, [r1] // r1 = a cmp r0, r1 // x <= a ? ble then // Branch to "then" if x <= a // test if ( x >= b ) movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r0, [r0] // r0 = x movw r1, #:lower16:b movt r1, #:upper16:b // r1 = addr(b) ldr r1, [r1] // r1 = b cmp r0, r1 // x >= b ? blt else // Branch to "else" if x < b then: // x = x + 1 movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r1, [r0] // r1 = x add r1, r1, #1 // r1 = x+1 str r1, [r0] // x = x+1 b ifEnd // ** SKIP over the else part !!! *** else: // x = x - 1 movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r1, [r0] // r1 = x sub r1, r1, #1 // r1 = x-1 str r1, [r0] // x = x-1 ifEnd: .data x: .4byte 5 // Try x: 10 < x < 20 a: .4byte 10 b: .4byte 20 .end

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/6-if/if-or.s

    How to run the program:

    To compile:   as255 if-or               To run: use EGTAPI

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