University of Washington Data Structures in Memory! � Arrays � One ‐ dimensional � Multi ‐ dimensional (nested) M l i di i l ( d) � Multi ‐ level � Structs � Alignment � Unions 23 April 2012 Data Structures 1 University of Washington Data Structures in Assembly… � Arrays? � Strings? � Structs? 23 April 2012 Data Structures 2
University of Washington Array Allocation � Basic Principle � T A[N]; � Array of data type T and length N � Contiguously allocated region of N * sizeof(T) bytes C ti l ll t d i f N * i f(T) b t char string[12]; x x + 12 int val[5]; x x + 4 x + 8 x + 12 x + 16 x + 20 double a[3]; x x + 8 x + 16 x + 24 char *p[3]; IA32 x x + 4 x + 8 x + 12 x86 ‐ 64 x x + 8 x + 16 x + 24 23 April 2012 Data Structures 3 University of Washington Array Access � Basic Principle � T A[N]; � Array of data type T and length N � Identifier A can be used as a pointer to array element 0: Type T* Id tifi A b d i t t l t 0 T T* 9 8 1 9 5 int val[5]; x x + 4 x + 8 x + 12 x + 16 x + 20 � Reference Type Value � val[4] int � val int * � val+1 int * � &val[2] int * � val[5] int � *(val+1) int � val + i int * 23 April 2012 Data Structures 4
University of Washington Array Access � Basic Principle � T A[N]; � Array of data type T and length N � Identifier A can be used as a pointer to array element 0: Type T* Id tifi A b d i t t l t 0 T T* 9 8 1 9 5 int val[5]; x x + 4 x + 8 x + 12 x + 16 x + 20 � Reference Type Value � val[4] int 5 � val int * x � val+1 int * x + 4 � &val[2] int * x + 8 � val[5] int ?? � *(val+1) int 8 � val + i int * x + 4 i 23 April 2012 Data Structures 5 University of Washington Array Example typedef int zip_dig[5]; zip_dig cmu = { 1, 5, 2, 1, 3 }; zip dig uw zip_dig uw = { 9, 8, 1, 9, 5 }; { 9, 8, 1, 9, 5 }; zip_dig ucb = { 9, 4, 7, 2, 0 }; 23 April 2012 Data Structures 6
University of Washington Array Example typedef int zip_dig[5]; zip_dig cmu = { 1, 5, 2, 1, 3 }; zip dig uw zip_dig uw = { 9, 8, 1, 9, 5 }; { 9, 8, 1, 9, 5 }; zip_dig ucb = { 9, 4, 7, 2, 0 }; 1 5 2 1 3 zip_dig cmu; 16 20 24 28 32 36 9 8 1 9 5 zip_dig uw ; 36 40 44 48 52 56 9 4 7 2 0 zip_dig ucb; 56 60 64 68 72 76 Declaration “ zip_dig uw ” equivalent to “ int uw[5] ” � Example arrays were allocated in successive 20 byte blocks � � Not guaranteed to happen in general 23 April 2012 Data Structures 7 University of Washington Array Accessing Example 9 8 1 9 5 zip_dig uw; 36 40 44 48 52 56 int get_digit (zip_dig z, int dig) { return z[dig]; � Register %edx contains } starting address of array IA32 IA32 � Register %eax contains array index # %edx = z # %eax = dig � Desired digit at movl (%edx,%eax,4),%eax # z[dig] 4*%eax + %edx � Use memory reference (%edx,%eax,4) 23 April 2012 8
University of Washington Referencing Examples 1 5 2 1 3 zip_dig cmu; 16 20 24 28 32 36 9 8 1 9 5 zip_dig uw ; 36 40 44 48 52 56 9 4 7 2 0 zip_dig ucb; 56 60 64 68 72 76 � Reference Address Value Guaranteed? uw[3] 36 + 4* 3 = 48 9 uw[6] [6] 36 + 4* 6 36 + 4* 6 = 60 60 4 4 What are these values? uw[-1] 36 + 4*-1 = 32 3 cmu[15] 16 + 4*15 = 76 ?? 23 April 2012 Data Structures 9 University of Washington Referencing Examples 1 5 2 1 3 zip_dig cmu; 16 20 24 28 32 36 9 8 1 9 5 zip_dig uw ; 36 40 44 48 52 56 9 4 7 2 0 zip_dig ucb; 56 60 64 68 72 76 � Reference Address Value Guaranteed? uw[3] 36 + 4* 3 = 48 9 uw[6] [6] 36 + 4* 6 36 + 4* 6 = 60 60 4 4 uw[-1] 36 + 4*-1 = 32 3 cmu[15] 16 + 4*15 = 76 ?? � No bound checking � Out ‐ of ‐ range behavior implementation ‐ dependent � No guaranteed relative allocation of different arrays 23 April 2012 Data Structures 10
University of Washington Referencing Examples 1 5 2 1 3 zip_dig cmu; 16 20 24 28 32 36 9 8 1 9 5 zip_dig uw ; 36 40 44 48 52 56 9 4 7 2 0 zip_dig ucb; 56 60 64 68 72 76 � Reference Address Value Guaranteed? uw[3] 36 + 4* 3 = 48 9 Yes No No uw[6] [6] 36 + 4* 6 = 60 36 + 4* 6 60 4 4 No uw[-1] 36 + 4*-1 = 32 3 No cmu[15] 16 + 4*15 = 76 ?? � No bound checking � Out ‐ of ‐ range behavior implementation ‐ dependent � No guaranteed relative allocation of different arrays 23 April 2012 Data Structures 11 University of Washington Array Loop Example int zd2int(zip_dig z) { int i; int zi = 0; for (i = 0; i < 5; i++) { for (i = 0; i < 5; i++) { zi = 10 * zi + z[i]; } return zi; } 23 April 2012 Data Structures 12
University of Washington Array Loop Example int zd2int(zip_dig z) { � Original int i; int zi = 0; for (i = 0; i < 5; i++) { for (i = 0; i < 5; i++) { zi = 10 * zi + z[i]; } return zi; } � Transformed int zd2int(zip_dig z) � Eliminate loop variable i { int zi = 0; � Convert array code to � Convert array code to int *zend = z + 4; pointer code do { � Express in do ‐ while form zi = 10 * zi + *z; (no test at entrance) z++; } while (z <= zend); return zi; } 23 April 2012 Data Structures 13 University of Washington Array Loop Implementation (IA32) int zd2int(zip_dig z) { int zi = 0; int *zend = z + 4; do { do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi; } # %ecx = z xorl %eax,%eax # zi = 0 leal 16(%ecx),%ebx l l 16(% ) % b # # zend d = z+4 +4 .L59: leal (%eax,%eax,4),%edx # 5*zi Translation? movl (%ecx),%eax # *z addl $4,%ecx # z++ leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi) cmpl %ebx,%ecx # z : zend jle .L59 # if <= goto loop 23 April 2012 Data Structures 14
University of Washington Array Loop Implementation (IA32) int zd2int(zip_dig z) int zd2int(zip_dig z) int zd2int(zip_dig z) int zd2int(zip_dig z) int zd2int(zip_dig z) � Registers { { { { { %ecx z int zi = 0; int zi = 0; int zi = 0; int zi = 0; int zi = 0; %eax zi int *zend = z + 4; int *zend = z + 4; int *zend = z + 4; int *zend = z + 4; int *zend = z + 4; %ebx zend do { do { do { do { do { do { do { do { do { do { zi = 10 * zi + *z; zi = 10 * zi + *z; zi = 10 * zi + *z; zi = 10 * zi + *z; zi = 10 * zi + *z; � Computations z++; z++; z++; z++; z++; � 10*zi + *z implemented as } while(z <= zend); } while(z <= zend); } while(z <= zend); } while(z <= zend); } while(z <= zend); *z + 2*(zi+4*zi) return zi; return zi; return zi; return zi; return zi; � z++ increments by 4 } } } } } # %ecx = z # %ecx = z # %ecx = z # %ecx = z # %ecx = z xorl %eax,%eax xorl %eax,%eax xorl %eax,%eax xorl %eax,%eax xorl %eax,%eax # zi = 0 # zi = 0 # zi = 0 # zi = 0 # zi = 0 leal 16(%ecx),%ebx leal 16(%ecx),%ebx l l leal 16(%ecx),%ebx leal 16(%ecx),%ebx l leal 16(%ecx),%ebx l l l 16(% l 16(% l 16(% l 16(% l 16(% ) % b ) % b ) % b ) % b ) % b # zend = z+4 # # # zend = z+4 # zend = z+4 # # # # zend # zend = z+4 d d d d d = z+4 +4 +4 +4 +4 +4 .L59: .L59: .L59: .L59: .L59: leal (%eax,%eax,4),%edx # 5*zi leal (%eax,%eax,4),%edx # 5*zi leal (%eax,%eax,4),%edx # 5*zi leal (%eax,%eax,4),%edx # 5*zi leal (%eax,%eax,4),%edx # 5*zi movl (%ecx),%eax movl (%ecx),%eax movl (%ecx),%eax movl (%ecx),%eax movl (%ecx),%eax # *z # *z # *z # *z # *z addl $4,%ecx addl $4,%ecx addl $4,%ecx addl $4,%ecx addl $4,%ecx # z++ # z++ # z++ # z++ # z++ leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi) leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi) leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi) leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi) leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi) cmpl %ebx,%ecx cmpl %ebx,%ecx cmpl %ebx,%ecx cmpl %ebx,%ecx cmpl %ebx,%ecx # z : zend # z : zend # z : zend # z : zend # z : zend jle .L59 jle .L59 jle .L59 jle .L59 jle .L59 # if <= goto loop # if <= goto loop # if <= goto loop # if <= goto loop # if <= goto loop 23 April 2012 Data Structures 15 University of Washington Nested Array Example #define PCOUNT 4 zip_dig sea[PCOUNT] = {{ 9, 8, 1, 9, 5 }, { 9, 8, 1, 0, 5 }, { 9 8 1 0 5 } { 9, 8, 1, 0, 3 }, { 9, 8, 1, 1, 5 }}; 23 April 2012 Data Structures 16
University of Washington Nested Array Example #define PCOUNT 4 zip_dig sea[PCOUNT] = {{ 9, 8, 1, 9, 5 }, { 9, 8, 1, 0, 5 }, { 9 8 1 0 5 } { 9, 8, 1, 0, 3 }, { 9, 8, 1, 1, 5 }}; 9 8 1 9 5 9 8 1 0 5 9 8 1 0 3 9 8 1 1 5 76 76 96 96 116 116 136 136 156 156 23 April 2012 Data Structures 17 University of Washington Nested Array Example #define PCOUNT 4 zip_dig sea[PCOUNT] = {{ 9, 8, 1, 9, 5 }, { 9 { 9, 8, 1, 0, 5 }, 8 1 0 5 } &sea[3][2]; { 9, 8, 1, 0, 3 }, { 9, 8, 1, 1, 5 }}; 9 8 1 9 5 9 8 1 0 5 9 8 1 0 3 9 8 1 1 5 76 76 96 96 116 116 136 136 156 156 � “row ‐ major” ordering of all elements � Guaranteed? 23 April 2012 Data Structures 18
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