In a C compiler, function calls are difficult to implement

correctly because they must respect a platform-specific calling

convention. But they are governed by a simple invariant:

parameters passed to a function must be received unaltered.

A violation of this invariant signals an inconsistency in a

compiler. We automatically test the consistency of C compilers

using randomly generated programs. An inconsistency

manifests itself as an assertion failure when compiling and

running the generated code. The generation of programs is

type-directed and can be controlled by the user with composable

random generators in about 100 lines of Lua. Lua

is a scr ipting language built into our testing tool that drives

program generation. Random testing is fully automatic, requires

no specification, yet is comparable in effectiveness

with specification-based testing from prior work. Using this

method, we uncovered 13 new bugs in mature open-source

and commercial C compilers.