Re: Add support to trace comparison instructions and switch statements

[Jakub Jelinek <jakub@redhat.com>]

On Sun, Sep 03, 2017 at 10:50:16AM +0200, Dmitry Vyukov wrote:
> What we instrument in LLVM is _comparisons_ rather than control
> structures. So that would be:
>     _4 = x_8(D) == 98;
> For example, result of the comparison can be stored into a bool struct
> field, and then used in branching long time after. We still want to
> intercept this comparison.

Then we need to instrument not just GIMPLE_COND, which is the stmt
where the comparison decides to which of the two basic block successors to
jump, but also GIMPLE_ASSIGN with tcc_comparison class
gimple_assign_rhs_code (the comparison above), and maybe also
GIMPLE_ASSIGN with COND_EXPR comparison code (that is say
  _4 = x_1 == y_2 ? 23 : _3;
).

> > Perhaps for -fsanitize-coverage= it might be a good idea to force
> > LOGICAL_OP_NON_SHORT_CIRCUIT/BRANCH_COST or whatever affects GIMPLE
> > decisions mentioned above so that the IL is closer to what the user wrote.
> 
> If we recurse down to comparison operations and instrument them, this
> will not be so important, right?

Well, if you just handle tcc_comparison GIMPLE_ASSIGN and not GIMPLE_COND,
then you don't handle many comparisons from the source code.  And if you
handle both, some of the GIMPLE_CONDs might be just artificial comparisons.
By pretending small branch cost for the tracing case you get fewer
artificial comparisons.

	Jakub