On Thu, 7 Nov 2013, Uros Bizjak wrote: > > I see code of the form (testing compilation rather than execution): > > > > flds 4(%esp) > > flds 8(%esp) > > fmulp %st, %st(1) > > fstps 12(%esp) > > > > where the fstps should result in the exception, and glibc uses volatile in > > several places, conditional on __FLT_EVAL_METHOD__ != 0, to force a > > conversion to the semantic type (whether for correct results, or to ensure > > exceptions). > > Yes, this is the exact sequence my example compiles to: > > 8048405: d9 44 24 14 flds 0x14(%esp) > 8048409: d9 44 24 18 flds 0x18(%esp) > 804840d: de c9 fmulp %st,%st(1) > 804840f: d9 5c 24 1c fstps 0x1c(%esp) > > unfortunately, it won't generate exception.
Are you sure? It's documented as generating an exception. That may mean, as usual on x87, setting the exception bit (as can be tested by fetestexcept) and only calling a trap handler on the *next* x87 instruction. So if fstps is the last floating-point instruction executed by the program, a trap handler may not be called - but that's no different from an ordinary floating-point compound assignment having the exception-raising operation as the last floating-point instruction. -- Joseph S. Myers jos...@codesourcery.com
