CS计算机代考程序代写 assembly js x86 Introduction to Computer Systems 15-213/18-243, spring 2009

Introduction to Computer Systems 15-213/18-243, spring 2009

CSE 2421
X86-64 Assembly Language Part 3: Control (Loops)

Today
Control: Condition codes (Review)
Conditional branches (Review)
Loops
Switch Statements

Jumping
jX Instructions
Jump to different part of code depending on condition codes

This is only a partial list
jX Condition Description
jmp 1 Unconditional
je ZF Equal / Zero
jne ~ZF Not Equal / Not Zero
js SF Negative
jns ~SF Nonnegative
jg ~(SF^OF)&~ZF Greater (Signed)
jge ~(SF^OF) Greater or Equal (Signed)
jl (SF^OF) Less (Signed)
jle (SF^OF)|ZF Less or Equal (Signed)
ja ~CF&~ZF Above (unsigned)
jb CF Below (unsigned)

Conditional Moves
cmovX Instructions
Move a value (or not) depending on condition codes
cmovX Condition Description
cmove ZF Equal / Zero
cmovne ~ZF Not Equal / Not Zero
cmovs SF Negative
cmovns ~SF Nonnegative
cmovg ~(SF^OF)&~ZF Greater (Signed)
cmovge ~(SF^OF) Greater or Equal (Signed)
cmovl (SF^OF) Less (Signed)
cmovle (SF^OF)|ZF Less or Equal (Signed)
cmova ~CF&~ZF Above (unsigned)
cmovb CF Below (unsigned)

Conditional Branch Example
C code example with assembly
long absdiff
(long x, long y)
{
long result;
if (x > y)
result = x-y;
else
result = y-x;
return result;
}
absdiff:
cmpq %rsi, %rdi # x:y
jle reverse
movq %rdi, %rax
subq %rsi, %rax
ret
reverse: # x <= y movq %rsi, %rax subq %rdi, %rax ret Register Use(s) %rdi Argument x %rsi Argument y %rax Return value Expressing with Goto Code C allows goto statement Jump to position designated by label long absdiff (long x, long y) { long result; if (x > y)
result = x-y;
else
result = y-x;
return result;
}
long absdiff_j
(long x, long y)
{
long result;
int ntest = x <= y; if (ntest) goto Else; result = x-y; goto Done; Else: result = y-x; Done: return result; } C Code val = Test ? Then_Expr : Else_Expr; Goto Version ntest = !Test; if (ntest) goto Else; val = Then_Expr; goto Done; Else: val = Else_Expr; Done: . . . General Conditional Expression Translation (Using Branches) Create separate code regions for then & else expressions Execute appropriate one val = x>y ? x-y : y-x;

C Code
val = Test
? Then_Expr
: Else_Expr;
Goto Version
result = Then_Expr;
eval = Else_Expr;
nt = !Test;
if (nt) result = eval;
return result;
Using Conditional Moves
Conditional Move Instructions
Instruction supports:
if (Test) Dest  Src
Supported in post-1995 x86 processors
GCC tries to use them
But, only when known to be safe
Why?
Branches are very disruptive to instruction flow through pipelines
Conditional moves do not require control transfer

Conditional Move Example
absdiff:
movq %rdi, %rax # x
subq %rsi, %rax # result = x-y
movq %rsi, %rdx
subq %rdi, %rdx # eval = y-x
cmpq %rsi, %rdi # x:y
cmovle %rdx, %rax # if <=, result = eval ret long absdiff (long x, long y) { long result; if (x > y)
result = x-y;
else
result = y-x;
return result;
}
Register Use(s)
%rdi Argument x
%rsi Argument y
%rax Return value

Expensive Computations
Bad Cases for Conditional Move
Both values get computed
Only makes sense when computations are very simple
val = Test(x) ? Hard1(x) : Hard2(x);
Risky Computations
Both values get computed
May have undesirable effects
val = p ? *p : 0;
Computations with side effects
Both values get computed
Must be side-effect free
val = x > 0 ? x*=7 : x+=3;

Today
Control: Condition codes (Review)
Conditional branches (Review)
Loops
Switch Statements

C Code
long pcount_do
(unsigned long x) {
long result = 0;
do {
result += x & 0x1;
x >>= 1;
} while (x);
return result;
}
C Code goto Version
long pcount_goto
(unsigned long x) {
long result = 0;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
return result;
}
“Do-While” Loop Example
Count number of 1’s in argument x (“popcount”)
Use conditional branch to either continue looping or to exit loop

C Code goto Version
“Do-While” Loop Compilation
movq $0, %rax # result = 0
loop: # loop:
movq %rdi, %rdx # t1 = x
andq $1, %rdx # t1 = t1 & 0x1
addq %rdx, %rax # result += t1
shrq %rdi # x >>= 1
jne loop # if (x) goto loop
ret # ret
long pcount_goto
(unsigned long x) {
long result = 0;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
return result;
}
Register Use(s)
%rdi Argument x
%rax result

C Code
do
Body
while (Test);
Goto Version
loop:
Body
if (Test)
goto loop
General “Do-While” Translation
Body:

{
Statement1;
Statement2;

Statementn;
}

While version
while (Test)
Body
General “While” Translation #1
“Jump-to-middle” translation
Used with gcc -Og
Goto Version
goto test;
loop:
Body
test:
if (Test)
goto loop;
done:

C Code
long pcount_while
(unsigned long x) {
long result = 0;
while (x) {
result += x & 0x1;
x >>= 1;
}
return result;
}
Jump to Middle Version
long pcount_goto_jtm
(unsigned long x) {
long result = 0;
goto test;
loop:
result += x & 0x1;
x >>= 1;
test:
if(x) goto loop;
return result;
}
While Loop Example #1
Compare to do-while version of function
Initial goto starts loop at test

While version
while (Test)
Body
Do-While Version
if (!Test)
goto done;
do
Body
while(Test);
done:
General “While” Translation #2
“Do-while” conversion
Used with gcc -O1
Goto Version
if (!Test)
goto done;
loop:
Body
if (Test)
goto loop;
done:

C Code
long pcount_while
(unsigned long x) {
long result = 0;
while (x) {
result += x & 0x1;
x >>= 1;
}
return result;
}
Do-While Version
long pcount_goto_dw
(unsigned long x) {
long result = 0;
if (!x) goto done;
loop:
result += x & 0x1;
x >>= 1;
if(x) goto loop;
done:
return result;
}
While Loop Example #2
Compare to do-while version of function
Initial conditional guards entrance to loop

“For” Loop Form
for (Init; Test; Update )
Body
General Form

#define WSIZE 8*sizeof(int)
long pcount_for
(unsigned long x)
{
size_t i;
long result = 0;
for (i = 0; i < WSIZE; i++) { unsigned bit = (x >> i) & 0x1;
result += bit;
}
return result;
}
i = 0
i < WSIZE i++ { unsigned bit = (x >> i) & 0x1;
result += bit;
}
Init

Test

Update

Body

“For” Loop  While Loop
for (Init; Test; Update )
Body
For Version

Init;
while (Test ) {
Body
Update;
}
While Version

For-While Conversion
long pcount_for_while
(unsigned long x)
{
size_t i;
long result = 0;
i = 0;
while (i < WSIZE) { unsigned bit = (x >> i) & 0x1;
result += bit;
i++;
}
return result;
}
i = 0
i < WSIZE i++ { unsigned bit = (x >> i) & 0x1;
result += bit;
}
Init

Test

Update
Body

C Code
“For” Loop Do-While Conversion
Initial test can be optimized away
long pcount_for
(unsigned long x)
{
size_t i;
long result = 0;
for (i = 0; i < WSIZE; i++) { unsigned bit = (x >> i) & 0x1;
result += bit;
}
return result;
}
Goto Version
long pcount_for_goto_dw
(unsigned long x) {
size_t i;
long result = 0;
i = 0;
if (!(i < WSIZE)) goto done; loop: { unsigned bit = (x >> i) & 0x1;
result += bit;
}
i++;
if (i < WSIZE) goto loop; done: return result; } Init !Test Body Update Test Today Control: Condition codes (Review) Conditional branches (Review) Loops Switch Statements Switch Statement Example Multiple case labels Here: 5 & 6 Fall through cases Here: 2 Missing cases Here: 4 long switch_eg (long x, long y, long z) { long w = 1; switch(x) { case 1: w = y*z; break; case 2: w = y/z; /* Fall Through */ case 3: w += z; break; case 5: case 6: w -= z; break; default: w = 2; } return w; } Jump Table Structure Code Block 0 Targ0: Code Block 1 Targ1: Code Block 2 Targ2: Code Block n–1 Targn-1: • • • Targ0 Targ1 Targ2 Targn-1 • • • jtab: goto *JTab[x]; switch(x) { case val_0: Block 0 case val_1: Block 1 • • • case val_n-1: Block n–1 } Switch Form Translation (Extended C) Jump Table Jump Targets Switch Statement Example Setup: long switch_eg(long x, long y, long z) { long w = 1; switch(x) { . . . } return w; } switch_eg: movq %rdx, %rcx cmpq $6, %rdi # x:6 ja .L8 jmp *.L4(,%rdi,8) What range of values takes default? Note that w not initialized here Register Use(s) %rdi Argument x %rsi Argument y %rdx Argument z %rax Return value Switch Statement Example long switch_eg(long x, long y, long z) { long w = 1; switch(x) { . . . } return w; } Indirect jump Jump table .section .rodata .align 8 .L4: .quad .L8 # x = 0 .quad .L3 # x = 1 .quad .L5 # x = 2 .quad .L9 # x = 3 .quad .L8 # x = 4 .quad .L7 # x = 5 .quad .L7 # x = 6 Setup: switch_eg: movq %rdx, %rcx cmpq $6, %rdi # x:6 ja .L8 # Use default jmp *.L4(,%rdi,8) # goto *JTab[x] Assembly Setup Explanation Table Structure Each target requires 8 bytes Base address at .L4 Jumping Direct: jmp .L8 Jump target is denoted by label .L8 Indirect: jmp *.L4(,%rdi,8) Start of jump table: .L4 Must scale by factor of 8 (addresses are 8 bytes) Fetch target from effective Address .L4 + x*8 Only for 0 ≤ x ≤ 6 Jump table .section .rodata .align 8 .L4: .quad .L8 # x = 0 .quad .L3 # x = 1 .quad .L5 # x = 2 .quad .L9 # x = 3 .quad .L8 # x = 4 .quad .L7 # x = 5 .quad .L7 # x = 6 .section .rodata .align 8 .L4: .quad .L8 # x = 0 .quad .L3 # x = 1 .quad .L5 # x = 2 .quad .L9 # x = 3 .quad .L8 # x = 4 .quad .L7 # x = 5 .quad .L7 # x = 6 Jump Table Jump table switch(x) { case 1: // .L3 w = y*z; break; case 2: // .L5 w = y/z; /* Fall Through */ case 3: // .L9 w += z; break; case 5: case 6: // .L7 w -= z; break; default: // .L8 w = 2; } Code Blocks (x == 1) .L3: movq %rsi, %rax # y imulq %rdx, %rax # y*z ret switch(x) { case 1: // .L3 w = y*z; break; . . . } Register Use(s) %rdi Argument x %rsi Argument y %rdx Argument z %rax Return value Handling Fall-Through long w = 1; . . . switch(x) { . . . case 2: w = y/z; /* Fall Through */ case 3: w += z; break; . . . } case 3: w = 1; case 2: w = y/z; goto merge; merge: w += z;