(mis)优化
寄存器的读/写非常特殊。我甚至敢说它们是副作用的体现。在前面的示例中，我们将四个不同的值写入同一个寄存器。
如果您不知道地址是一个寄存器，那么您可能已经简化了逻辑，只将最终值1 << (11 + 16)写入寄存器。
实际上，编译器的后端/优化器LLVM不知道我们正在处理寄存器，并将合并写入，从而改变程序的行为。让我们快速检查一下。
$ cargo run --release
(..)
Breakpoint 1, registers::__cortex_m_rt_main_trampoline () at src/07-registers/src/main.rs:7
7 #[entry]
(gdb) step
registers::__cortex_m_rt_main () at src/07-registers/src/main.rs:9
9 aux7::init();
(gdb) next
25 *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
(gdb) disassemble /m
Dump of assembler code for function _ZN9registers18__cortex_m_rt_main17h45b1ef53e18aa8d0E:
8 fn main() -> ! {
0x08000248 <+0>: push {r7, lr}
0x0800024a <+2>: mov r7, sp
9 aux7::init();
0x0800024c <+4>: bl 0x8000260 <aux7::init>
0x08000250 <+8>: movw r0, #4120 ; 0x1018
0x08000254 <+12>: mov.w r1, #134217728 ; 0x8000000
0x08000258 <+16>: movt r0, #18432 ; 0x4800
10
11 unsafe {
12 // A magic address!
13 const GPIOE_BSRR: u32 = 0x48001018;
14
15 // Turn on the "North" LED (red)
16 *(GPIOE_BSRR as *mut u32) = 1 << 9;
17
18 // Turn on the "East" LED (green)
19 *(GPIOE_BSRR as *mut u32) = 1 << 11;
20
21 // Turn off the "North" LED
22 *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);
23
24 // Turn off the "East" LED
25 *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
=> 0x0800025c <+20>: str r1, [r0, #0]
0x0800025e <+22>: b.n 0x800025e <registers::__cortex_m_rt_main+22>
End of assembler dump.
这次LED的状态没有改变！str指令是将值写入寄存器的指令。我们的debug（未优化）程序有四个，每
个写入寄存器一个，但release（优化）程序只有一个。
我们可以使用objdump检查并将输出捕获到out.asm：
# same as cargo objdump -- -d --no-show-raw-insn --print-imm-hex --source target/thumbv7em-none-eabihf/debug/registers
cargo objdump --bin registers -- -d --no-show-raw-insn --print-imm-hex --source > debug.txt
然后检查debug.txt查找main我们看到4个str指令：
080001ec <main>:
; #[entry]
80001ec: push {r7, lr}
80001ee: mov r7, sp
80001f0: bl #0x2
80001f4: trap
080001f6 <registers::__cortex_m_rt_main::hc2e3436fa38cd6f2>:
; fn main() -> ! {
80001f6: push {r7, lr}
80001f8: mov r7, sp
; aux7::init();
80001fa: bl #0x3e
80001fe: b #-0x2 <registers::__cortex_m_rt_main::hc2e3436fa38cd6f2+0xa>
; *(GPIOE_BSRR as *mut u32) = 1 << 9;
8000200: movw r0, #0x2640
8000204: movt r0, #0x800
8000208: ldr r0, [r0]
800020a: movw r1, #0x1018
800020e: movt r1, #0x4800
8000212: str r0, [r1]
; *(GPIOE_BSRR as *mut u32) = 1 << 11;
8000214: movw r0, #0x2648
8000218: movt r0, #0x800
800021c: ldr r0, [r0]
800021e: str r0, [r1]
; *(GPIOE_BSRR as *mut u32) = 1 << (9 + 16);
8000220: movw r0, #0x2650
8000224: movt r0, #0x800
8000228: ldr r0, [r0]
800022a: str r0, [r1]
; *(GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
800022c: movw r0, #0x2638
8000230: movt r0, #0x800
8000234: ldr r0, [r0]
8000236: str r0, [r1]
; loop {}
8000238: b #-0x2 <registers::__cortex_m_rt_main::hc2e3436fa38cd6f2+0x44>
800023a: b #-0x4 <registers::__cortex_m_rt_main::hc2e3436fa38cd6f2+0x44>
(..)
我们如何防止LLVM错误优化我们的程序？我们使用易失性操作而不是简单的读/写：
#![no_main] #![no_std] use core::ptr; #[allow(unused_imports)] use aux7::entry; #[entry] fn main() -> ! { aux7::init(); unsafe { // A magic address! const GPIOE_BSRR: u32 = 0x48001018; // Turn on the "North" LED (red) ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 9); // Turn on the "East" LED (green) ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 11); // Turn off the "North" LED ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (9 + 16)); // Turn off the "East" LED ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (11 + 16)); } loop {} }
生成release.txt使用--release模式。
cargo objdump --release --bin registers -- -d --no-show-raw-insn --print-imm-hex --source > release.txt
现在在release.txt中找到main程序，我们看到了4个str指令。
0800023e <main>:
; #[entry]
800023e: push {r7, lr}
8000240: mov r7, sp
8000242: bl #0x2
8000246: trap
08000248 <registers::__cortex_m_rt_main::h45b1ef53e18aa8d0>:
; fn main() -> ! {
8000248: push {r7, lr}
800024a: mov r7, sp
; aux7::init();
800024c: bl #0x22
8000250: movw r0, #0x1018
8000254: mov.w r1, #0x200
8000258: movt r0, #0x4800
; intrinsics::volatile_store(dst, src);
800025c: str r1, [r0]
800025e: mov.w r1, #0x800
8000262: str r1, [r0]
8000264: mov.w r1, #0x2000000
8000268: str r1, [r0]
800026a: mov.w r1, #0x8000000
800026e: str r1, [r0]
8000270: b #-0x4 <registers::__cortex_m_rt_main::h45b1ef53e18aa8d0+0x28>
(..)
我们看到四个写入 (str指令)被保留。如果您使用gdb运行它，您还会看到我们得到了预期的行为。
注意：下一个
next将无休止地执行loop {}，使用Ctrl-c返回(gdb)提示符。
$ cargo run --release
(..)
Breakpoint 1, registers::__cortex_m_rt_main_trampoline () at src/07-registers/src/main.rs:9
9 #[entry]
(gdb) step
registers::__cortex_m_rt_main () at src/07-registers/src/main.rs:11
11 aux7::init();
(gdb) next
18 ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 9);
(gdb) next
21 ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << 11);
(gdb) next
24 ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (9 + 16));
(gdb) next
27 ptr::write_volatile(GPIOE_BSRR as *mut u32, 1 << (11 + 16));
(gdb) next
^C
Program received signal SIGINT, Interrupt.
0x08000270 in registers::__cortex_m_rt_main ()
at ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1124
1124 intrinsics::volatile_store(dst, src);
(gdb)