(Not trying to promote any specific company, but just listing some commercial services that could be used to validate the compiler.)
For free testing, see Standards and Regression Testing.
ACE SuperTest C/C++ compiler test & validation suite
The SuperTest suite contains over sixteen thousand source files and provides over 800,000 quality and conformance tests.
The Plum Hall Validation Suite for C
Authoritative test suite for the ANSI/ISO C Standard, with both positive and negative tests for all requirements of that standard -- with over 56,000 lines of C source.
Glen McCluskey C99/C9X (ISO/IEC 9899:1999) Test Suite
Free samples are available. Seems that *.c0 are for compilation, *.r0 for running.
Tydeman's C99 FPCE Test Suite
The Floating-Point C Extensions test suite checks most parts of C99, with a special emphasis on floating-point (both binary and decimal) of both the math library and code generation. There are over 22,000 files with over 2,500,000 lines of code and test vectors. Free sample tests are available on the web site.