[Bug tree-optimization/68235] New: gimple optimisations always use global -fmath-errno setting

[rsandifo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68235

            Bug ID: 68235
           Summary: gimple optimisations always use global -fmath-errno
                    setting
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rsandifo at gcc dot gnu.org
  Target Milestone: ---

At the moment the ECF_* flags for a gimple call to a built-in
function are derived from the function decl, which in turn is
derived from the global command-line options.  So if the compiler
is run with -fno-math-errno, we always assume functions don't set
errno, regardless of local optimisation options.  Similarly if the
compiler is run with -fmath-errno, we always assume functions set errno.

This shows up in gcc.dg/lto/20110201-1_0.c, where we compile
the file with -O0 and use -O2 -ffast-math for a specific function.
-O2 -ffast-math is enough for us to convert cabs to sqrt as hoped,
but because of the global -fmath-errno setting, we assume that the
call to sqrt is not pure or const and create vops for it.  This makes
it appear to the gimple code that a simple sqrt optab isn't enough.