The following code transposes the elements of an MM array, where M is a constant defined by #define:

#define M ... void transpose( long A[M][M] ) { long i, j; for( i = 0; i  
When compiled with optimization level -O1, gcc generates the following code for the inner loop of the function:
 
.L6: movq (%rdx), %rcx # loop body starts here movq (%rax), %rsi movq %rsi, (%rdx) movq %rcx, (%rax) addq $8, %rdx # loop update code starts here addq $120, %rax cmpq %rdi, %rax jne .L6 
 
We can see that gcc has converted the array indexing to pointer code.
 
 Which register holds a pointer to array element A[i][j]?
 
 Which register holds a pointer to array element A[j][i]?
 
 What is the value of M (in decimal)? 
 
 
 
 (PS: If you're curious, the full assembly code is:)
 
 transpose: leaq 120(%rdi), %r10 leaq 8(%rdi), %r9 subq $-128, %rdi movl $0, %r8d jmp .L2 .L6: movq (%rdx), %rcx movq (%rax), %rsi movq %rsi, (%rdx) movq %rcx, (%rax) addq $8, %rdx addq $120, %rax cmpq %rdi, %rax jne .L6 .L5: addq $120, %r10 addq $8, %r9 subq $-128, %rdi .L2: addq $1, %r8 cmpq $15, %r8 je .L1 testq %r8, %r8 jle .L5 movq %r9, %rax movq %r10, %rdx jmp .L6 .L1: rep ret 
  MM