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gcc - 为什么相同的 CFLAGS 可以在 makefile 中重复两次？

问题描述

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

第一行与第三行完全相同。

注释第一行后，“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

我感到困惑。

标签： gccmakefilearmstm32cortex-m

解决方案

CFLAGS = -g -O2 -Wall

此行覆盖第一行（注意分配），因此，如果删除第三行，则在没有这些选项的情况下进行编译，并且应用工具链默认值。在这种非常特殊的情况下，编译器似乎不接受默认目标的源。

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