tailieunhanh - ARM System Developer’s Guide phần 3

Các hướng dẫn nhân dài (SMLAL, SMULL, UMLAL, và UMULL) tạo ra một kết quả 64-bit. Kết quả là quá lớn để phù hợp với một đăng ký 32-bit duy nhất để kết quả được đặt trong hai đăng ký dán nhãn RdLo và RdHi. RdLo nắm giữ thấp hơn 32 bit của các kết quả 64-bit, và RdHi nắm giữ 32 bit cao hơn của | 128 Chapter 5 Efficient C Programming In this case the second value of step is different from the first and has the value timer1. This forces the compiler to insert an extra load instruction. The same problem occurs if you use structure accesses rather than direct pointer access. The following code also compiles inefficiently typedef struct int step State typedef struct int timerl timer2 Timers void timers_v2 State state Timers timers timers- timer1 state- step timers- timer2 state- step The compiler evaluates state- step twice in case state- step and timers- timer1 are at the same memory address. The fix is easy Create a new local variable to hold the value of state- step so the compiler only performs a single load. Example In the code for timers_v3 we use a local variable step to hold the value of state- step. Now the compiler does not need to worry that state may alias with timers. void timers_v3 State state Timers timers int step state- step timers- timer1 step timers- timer2 step You must also be careful of other less obvious situations where aliasing may occur. When you call another function this function may alter the state of memory and so change the values of any expressions involving memory reads. The compiler will evaluate the expressions again. For example suppose you read state- step call a function and then read state- step again. The compiler must assume that the function could change the value of state- step in memory. Therefore it will perform two reads rather than reusing the first value it read for state- step. Another pitfall is to take the address of a local variable. Once you do this the variable is referenced by a pointer and so aliasing can occur with other pointers. The compiler is likely to keep reading the variable from the stack in case aliasing occurs. Consider the following example which reads and then checksums a data packet int checksum_next_packet void int data int N sum 0 Pointer Aliasing 129 data get_next_packet N do sum .