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17.1 The Global targetm Variable

— Variable: struct gcc_target targetm

The target .c file must define the global targetm variable which contains pointers to functions and data relating to the target machine. The variable is declared in target.h; target-def.h defines the macro TARGET_INITIALIZER which is used to initialize the variable, and macros for the default initializers for elements of the structure. The .c file should override those macros for which the default definition is inappropriate. For example: 

          #include "target.h"
          #include "target-def.h"
          
          /* Initialize the GCC target structure.  */
          
          #undef TARGET_COMP_TYPE_ATTRIBUTES
          #define TARGET_COMP_TYPE_ATTRIBUTES machine_comp_type_attributes
          
          struct gcc_target targetm = TARGET_INITIALIZER;

Where a macro should be defined in the .c file in this manner to form part of the targetm structure, it is documented below as a “Target Hook” with a prototype. Many macros will change in future from being defined in the .h file to being part of the targetm structure.

Similarly, there is a targetcm variable for hooks that are specific to front ends for C-family languages, documented as “C Target Hook”. This is declared in c-family/c-target.h, the initializer TARGETCM_INITIALIZER in c-family/c-target-def.h. If targets initialize targetcm themselves, they should set target_has_targetcm=yes in config.gcc; otherwise a default definition is used.

Similarly, there is a targetm_common variable for hooks that are shared between the compiler driver and the compilers proper, documented as “Common Target Hook”. This is declared in common/common-target.h, the initializer TARGETM_COMMON_INITIALIZER in common/common-target-def.h. If targets initialize targetm_common themselves, they should set target_has_targetm_common=yes in config.gcc; otherwise a default definition is used.