为什么同一个 CFLAGS 可以在 makefile 中重复两次？

Question

这是 makefile 的部分：

CFLAGS += -mthumb -mcpu=cortex-m7 -mfloat-abi=hard -mfpu=fpv5-d16

CFLAGS  = -g -O2 -Wall
CFLAGS += -mthumb -mcpu=cortex-m7 -mfloat-abi=hard -mfpu=fpv5-d16

第一行和第三行完全一样

第一行注释后，"make"不会出错。

但是在注释第三行之后，"make"就出错了。错误：

/tmp/ccIrbEv9.s: Assembler messages:
/tmp/ccIrbEv9.s:77: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:368: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:402: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:1276: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:1305: Error: selected processor does not support `rbit r0,r3' in ARM mode
/tmp/ccIrbEv9.s:1371: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:1400: Error: selected processor does not support `rbit r0,r3' in ARM mode
/tmp/ccIrbEv9.s:1544: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:1573: Error: selected processor does not support `rbit r0,r3' in ARM mode
/tmp/ccIrbEv9.s:1643: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:1672: Error: selected processor does not support `rbit r0,r3' in ARM mode
/tmp/ccIrbEv9.s:2989: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:3021: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:3060: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:3096: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:3342: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:3371: Error: selected processor does not support `rbit r0,r3' in ARM mode
/tmp/ccIrbEv9.s:3443: Error: selected processor does not support `rbit r0,r0' in ARM mode
/tmp/ccIrbEv9.s:3472: Error: selected processor does not support `rbit r0,r3' in ARM mode
/tmp/ccIrbEv9.s:3783: Error: selected processor does not support `rbit r0,r0' in ARM mode
Makefile:27: recipe for target 'Src/stm32f7xx_hal_dma2d.o' failed

我感到很困惑。

Answer 1

CFLAGS = -g -O2 -Wall

此行会覆盖第一行（注意分配），因此，如果您删除第三行，您将在没有这些选项的情况下进行编译，并且会应用工具链默认值。在这种非常特殊的情况下，编译器似乎不接受默认目标的来源。

Answer 2

注意区别

CFLAGS += ...

和：

CFLAGS = ...

加号让一切变得不同。它通过附加标志修改 CFLAGS 值。没有加号，值被替换，之前的值丢失。

第一个CFLAGS += ...可以去掉，因为它的值被CFLAGS = ...代替了。
CFLAGS = ... 可以删除，因为代码在没有它们的情况下也能编译。
不能删除最后一个 CFLAGS += ...，因为它提供了对您的代码必不可少的编译器标志。

为什么同一个 CFLAGS 可以在 makefile 中重复两次？

Why the same CFLAGS can be repeated twice in a makefile?

gcc

arm

makefile

stm32

cortex-m