Translating a compound boolean expression
Compound boolean expressions use
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Example:
if (a <= x && x <= b)
x = x + 1;
else
x = x - 1;
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Compilers use the short-circuit boolean evaluation technique to translate compound boolean expressions
Short-circuit boolean evaluation technique for
&& expressions
The short-circuit boolean evaluation for the AND clause is based on the following:
condition1 && condition2: If condition1 = false then Outcome ≡ false // Outcome can be computed // with ONE condition If condition1 = true then Outcome ≡ condition2 |
We can construct a program flow for a compound && boolean expression using the above short-circuit boolean evaluation technique
Flow chart of
the AND condition
using short-circuit boolean evaluation
The short circuit boolean evaluation will result in the following flow chart:
If condition C1 ≡ false, then C1 && C2 ≡ false and the program can proceed to the ELSE part
But if condition C1 ≡ true, then C1 && C2 will depend on the outcome of C2.
Program flow of a
compound && condition
if ( cond1 && cond2 ) -----> |
statement1(s); V
else +--------------+ FALSE
statement2(s); | eval cond1 |-----------------+
+--------------+ |
| TRUE |
V |
+--------------+ FALSE |
| eval cond2 |------------+ |
+--------------+ | |
| TRUE | |
V | |
statement1(s) (then) | |
| | |
+<--------+ | |
| (else) | |
| statement2(s) <-----------+----+
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+-------->+
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V
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Programming example with compound
&& condition
Example && condition:
int x, y, a;
if (a <= x && x <= b)
x = x + 1;
else
x = x - 1;
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As before, I will omit discussing the variable definitions !
(I showed you the variable definitions to give you their data type information !)
Programming example with compound
&& condition
if (a <= x && x <= b)
x = x + 1;
else
x = x - 1;
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The corresponding flow chart is:
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Programming example with compound
&& condition
.text
main:
// Test if (a <= x )
movw r0, #:lower16:a
movt r0, #:upper16:a // r0 = addr(a)
ldr r0, [r0] // r0 = a
movw r1, #:lower16:x
movt r1, #:upper16:x // r1 = addr(x)
ldr r1, [r1] // r1 = x
cmp r0, r1 // a <= x ?
bgt else // Branch to "else" if a > x
// 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 ?
bgt 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:
/* --------------------------------------------------
Begin of the permanent program variables
-------------------------------------------------- */
.data
x: .4byte 15 // Try x outside 10 ... 20
a: .4byte 10
b: .4byte 20
.end
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DEMO: /home/cs255001/demo/asm/6-if/if-and.s