# The GHC Commentary - Spineless Tagless C

The C code generated by GHC doesn't use higher-level features of C to be able to control as precisely as possible what code is generated. Moreover, it uses special features of gcc (such as, first class labels) to produce more efficient code.

STG C makes ample use of C's macro language to define idioms, which also reduces the size of the generated C code (thus, reducing I/O times). These macros are defined in the C headers located in GHC's `includes` directory.

#### `TailCalls.h`

`TailCalls.h` defines how tail calls are implemented - and in particular - optimised in GHC generated code. The default case, for an architecture for which GHC is not optimised, is to use the mini interpreter described in the STG paper.

For supported architectures, various tricks are used to generate assembler implementing proper tail calls. On i386, gcc's first class labels are used to directly jump to a function pointer. Furthermore, markers of the form `--- BEGIN ---` and `--- END ---` are added to the assembly right after the function prologue and before the epilogue. These markers are used by the Evil Mangler.

Last modified: Wed Aug 8 19:28:29 EST 2001