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# On the other hand registers ebx esi and edi are

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Unformatted text preview: done: This, in turn, can be transformed into goto code as: 3.6. CONTROL init-expr; t = test-expr; if (!t) goto done; loop: body-statement update-expr; t = test-expr; if (t) goto loop; done: 127 As an example, the following code shows an implementation of the Fibonacci function using a for loop: code/asm/ﬁb.c 1 2 3 4 5 6 7 8 9 10 11 12 13 14 int fib_f(int n) { int i; int val = 1; int nval = 1; for (i = 1; i &lt; n; i++) { int t = val+nval; val = nval; nval = t; } return val; } code/asm/ﬁb.c The transformation of this code into the while loop form gives code identical to that for the function fib_w shown in Figure 3.13. In fact, GCC generates identical assembly code for the two functions. Practice Problem 3.12: The following assembly code: Initially x, y, and n are offsets 8, 12, and 16 from %ebp 1 2 3 4 5 6 movl 8(%ebp),%ebx movl 16(%ebp),%edx xorl %eax,%eax decl %edx js .L4 movl %ebx,%ecx 128 7 8 9 10 11 12 13 CHAPTER 3. MACHINE-LEVEL REPRESENTATION OF C PROGRAMS imull 12(%ebp),%ecx .p2align 4,,7 Inserted to optimize cache performance .L6: addl %ecx,%eax subl %ebx,%edx jns ....
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