为什么相同的CFLAGS可以在makefile中重复两次？

Question

这是makefile的片段：

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

第一行与第三行完全相同。

注释第一行后，“ make”没有错。

但是在注释第三行后，“ make”出了错。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 

我感到困惑。

Answer 1

CFLAGS = -g -O2 -Wall

此行将覆盖第一行（请注意分配），因此，如果删除第三行，则编译时将不使用这些选项，并且将应用工具链默认值。在这种非常特殊的情况下，似乎编译器不接受默认目标的源。

Answer 2

注意两者之间的区别

CFLAGS += ...

和：

CFLAGS = ...

加号使一切有所不同。它通过附加标志来修改CFLAGS值。如果没有加号，则该值将被替换，并且先前的值将丢失。

1. 第一个CFLAGS += ...可以被删除，因为它的值被CFLAGS = ...替换。

2. [CFLAGS = ...可以删除，因为代码也将在没有它们的情况下进行编译。

3. 最后一个CFLAGS += ...不能删除，因为它提供了对于代码必不可少的编译器标志。