SDCC

Text copied from the official SDCC site

What is SDCC?

SDCC is a Freeware, retargettable, optimizing ANSI - C language compiler that targets the Intel 8051, Maxim 80DS390, Zilog Z80 and the Motorola 68HC08 based MCUs. Work is in progress on supporting the Microchip PIC16 and PIC18 series. The entire source code for the compiler is distributed under GPL.

Some of the features include:

ASXXXX and ASLINK, a Freeware, retargettable assembler and linker.
extensive MCU specific language extensions, allowing effective use of the underlying hardware.
a host of standard optimizations such as global sub expression elimination, loop optimizations (loop invariant, strength reduction of induction variables and loop reversing ), constant folding and propagation, copy propagation, dead code elimination and jump tables for 'switch' statements.
MCU specific optimisations, including a global register allocator.
adaptable MCU specific backend that should be well suited for other 8 bit MCUs
independent rule based peep hole optimizer.
a full range of data types: char (8 bits, 1 byte), short (16 bits, 2 bytes), int (16 bits, 2 bytes), long (32 bit, 4 bytes) and float (4 byte IEEE).
the ability to add inline assembler code anywhere in a function.
the ability to report on the complexity of a function to help decide what should be re-written in assembler.
a good selection of automated regression tests.