Plan for today
Example question from previous final
Using gcc and gdb to generate and step through x86 code
Generating x86 instead of MIPS
–  Figuring out the x86 calling convention
–  Enough x86 to replace MIPS
Generating Java byte code instead of MIPS
–  Using javap and Jasmin to figure it out step-by-step
CS453 Lecture
x86 code gen
1
Instruction selection for x86-64
Registers
–  16 64-bit registers
–  RSP, the stack pointer register
–  RBP, the frame pointer register
–  32-bit register names used to access lower 32-bits of corresponding 64-bit register:
eax, edx, exc, ebx, esi, esi, edi, esp and ebp
Representations
–  Constants prefixed with ‘$’, for example $3, $4, $-5, etc
–  Registers prefixed with ‘%’, for example %rsp, %rbp, etc.
Some Instructions
–  movl %eax, -12(%rbp)
// M[%rbp-12] = %eax
–  addl -20(%rbp), %eax
// %eax = M[%rbp-20] * %eax
–  cmpl -4(%rbp), %eax
–  jge .L2
// if ( M[%rbp-4] >= %eax ) goto .L2
CS453 Lecture
x86 code gen
2
x86-64 example
.file
"funcCall2.c”
.text
.globl foo
.type
foo, @function
foo:
pushq
%rbp
movq
%rsp, %rbp
movl
%edi, -20(%rbp)
movl
%esi, -24(%rbp)
movl
-24(%rbp), %eax
addl
-20(%rbp), %eax
movl
%eax, -4(%rbp)
movl
-4(%rbp), %eax
leave
ret
.size
foo, .-foo
# %rsp = %rsp-8; M[%rsp ] = %rbp
# %rbp = %rsp
# storing parameters to stack
# accessing x
# adding y to x and storing in %eax
CS453 Lecture
x86 code gen
3
Example continued...
.globl main
.type
main, @function
main:
pushq
%rbp
# %rsp = %rsp-8; M[%rsp ] = %rbp;
pushes onto stack
movq
%rsp, %rbp # %rbp = %rsp
subq
$16, %rsp
# %rsp = %rsp - 16
movl
$5, %esi
# %esi = 5
movl
$4, %edi
# %edi = 4
call
foo
movl
%eax, -4(%rbp) # M[%rbp-4] = %eax
movl
-4(%rbp), %eax # %eax = M[%rbp-4]
leave
ret
CS453 Lecture
x86 code gen
4
Suggested Exercises
Compile the following three C programs
–  using ‘gcc -S filename.c’ on a 32-bit machine
–  read http://cs.wellesley.edu/~cs342/fall06/crash-course.pdf, pages 5-9
Answer the following questions (use google if necessary)
–  Identify and explain the lines of assembly code (5 lines that start at cmpl)
generated for the ‘if’ statement.
–  What is the return register? How do you know?
int main() {!
return 20;!
}!
CS453 Lecture
int main()
int x,
x = 2;
return
}!
{!
y;!
y = 42;!
x*y;!
x86 code gen
int main() {!
int x, y;!
x = 2; y = 42;!
if (x>y) {!
return 7;!
} else {!
return 8;!
}!
}!
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