In linux-5.4.21 code, To prevent some variables from being 'optimized out' during kernel code analysis using gdb, I added in drivers/iommu/Makefile, (replacing -O2 flags to -O0 flags for some files, I found http://www.joelfernandes.org/linux/2018/06/10/kernel-gdb.html ) CFLAGS_REMOVE_arm-smmu.o := -O2 CFLAGS_REMOVE_arm-smmu-impl.o := -O2 CFLAGS_REMOVE_io-pgtable-arm-v7s.o := -O2 CFLAGS_REMOVE_io-pgtable-arm.o := -O2 CFLAGS_REMOVE_of_iommu.o := -O2 CFLAGS_REMOVE_arm-smmu-v3.o := -O2 CFLAGS_arm-smmu.o := -O0 CFLAGS_arm-smmu-impl.o := -O0 CFLAGS_io-pgtable-arm-v7s.o := -O0 CFLAGS_io-pgtable-arm.o := -O0 CFLAGS_of_iommu.o := -O0 CFLAGS_arm-smmu-v3.o := -O0 But when I do make ARCH=arm64 CROSS_COMPILE=aarch64-none-elf- Image, I get CALL scripts/checksyscalls.sh CALL scripts/atomic/check-atomics.sh CHK include/generated/compile.h CC drivers/iommu/io-pgtable-arm.o In file included from ././include/linux/compiler_types.h:68, from <command-line>: ./arch/arm64/include/asm/jump_label.h: In function 'system_uses_lse_atomics': ./include/linux/compiler-gcc.h:120:38: warning: asm operand 0 probably doesn't match constraints 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0) | ^~~ ./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto' 38 | asm_volatile_goto( | ^~~~~~~~~~~~~~~~~ ./include/linux/compiler-gcc.h:120:38: warning: asm operand 0 probably doesn't match constraints 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0) | ^~~ ./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto' 38 | asm_volatile_goto( | ^~~~~~~~~~~~~~~~~ ./include/linux/compiler-gcc.h:120:38: error: impossible constraint in 'asm' 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0) | ^~~ ./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto' 38 | asm_volatile_goto( | ^~~~~~~~~~~~~~~~~ ./include/linux/compiler-gcc.h:120:38: error: impossible constraint in 'asm' 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0) | ^~~ ./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto' 38 | asm_volatile_goto( | ^~~~~~~~~~~~~~~~~ make[2]: *** [scripts/Makefile.build:266: drivers/iommu/io-pgtable-arm.o] Error 1 make[1]: *** [scripts/Makefile.build:509: drivers/iommu] Error 2 make: *** [Makefile:1652: drivers] Error 2 Does anybody know how can I use -O0 flag? (I tried -Og flag but some variables are still optimized out). Thank you! Chan Kim