#### 6.4.1 Idioms Used in Testsuite Code

In general, C testcases have a trailing `-n.c`, starting with `-1.c`, in case other testcases with similar names are added later. If the test is a test of some well-defined feature, it should have a name referring to that feature such as `feature-1.c`. If it does not test a well-defined feature but just happens to exercise a bug somewhere in the compiler, and a bug report has been filed for this bug in the GCC bug database, `prbug-number-1.c` is the appropriate form of name. Otherwise (for miscellaneous bugs not filed in the GCC bug database), and previously more generally, test cases are named after the date on which they were added. This allows people to tell at a glance whether a test failure is because of a recently found bug that has not yet been fixed, or whether it may be a regression, but does not give any other information about the bug or where discussion of it may be found. Some other language testsuites follow similar conventions.

In the `gcc.dg` testsuite, it is often necessary to test that an error is indeed a hard error and not just a warning—for example, where it is a constraint violation in the C standard, which must become an error with `-pedantic-errors`. The following idiom, where the first line shown is line line of the file and the line that generates the error, is used for this:

         /* { dg-bogus "warning" "warning in place of error" } */
         /* { dg-error "regexp" "message" { target *-*-* } line } */

It may be necessary to check that an expression is an integer constant expression and has a certain value. To check that E has value V, an idiom similar to the following is used:

         char x[((E) == (V) ? 1 : -1)];

In `gcc.dg` tests, `__typeof__` is sometimes used to make assertions about the types of expressions. See, for example, `gcc.dg/c99-condexpr-1.c`. The more subtle uses depend on the exact rules for the types of conditional expressions in the C standard; see, for example, `gcc.dg/c99-intconst-1.c`.

It is useful to be able to test that optimizations are being made properly. This cannot be done in all cases, but it can be done where the optimization will lead to code being optimized away (for example, where flow analysis or alias analysis should show that certain code cannot be called) or to functions not being called because they have been expanded as built-in functions. Such tests go in `gcc.c-torture/execute`. Where code should be optimized away, a call to a nonexistent function such as `link_failure ()` may be inserted; a definition

         #ifndef __OPTIMIZE__
         void
         link_failure (void)
         {
           abort ();
         }
         #endif

will also be needed so that linking still succeeds when the test is run without optimization. When all calls to a built-in function should have been optimized and no calls to the non-built-in version of the function should remain, that function may be defined as `static` to call `abort ()` (although redeclaring a function as static may not work on all targets).

All testcases must be portable. Target-specific testcases must have appropriate code to avoid causing failures on unsupported systems; unfortunately, the mechanisms for this differ by directory.

FIXME: discuss non-C testsuites here.