https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97653
Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Priority|P3 |P1 Target Milestone|--- |11.0 Summary|Incorrect long double |[11 Regression] Incorrect |calculation with |long double calculation |-mabi=ibmlongdouble |with -mabi=ibmlongdouble --- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> --- So, what I missed was that my gcc on gccfarm has been configured without the --with-long-double-format=ieee option, but Jonathan's gcc has been configured with that option. When compiling the #c9 testcase with -mabi=ibmlongdouble , both on compiler that was configured --with-long-double-format=ieee and on compiler that was configured without it, gcc emits calls to __floatunditf __gcc_qmul __fixunstfdi while when compiling the testcase with -mabi=ieeelongdouble , on both compilers it emits calls to __floatundikf __mulkf3 __fixunskfdi So far so good. The problem I see is that depending on how gcc has been configured, libgcc changes. In gcc configured without --with-long-double-format=ieee , I see __floatunditf calling __gcc_qadd and __gcc_qmul, so looks that the tf it talks about is IBM double double aka if. But looking at __floatunditf on --with-long-double-format=ieee configured gcc, I see it calls __floatundikf, __mulkf3 and __addkf3. So it looks both like ABI incompatibility and something else weird going on, because if it was treating the tf as kf, why would it call __floatundikf and something on top of that? Skimming other __*tf* functions in libgcc.a in --with-long-double-format=ieee configured gcc, __powitf2 calls __gcc_q{mul,div}, so might be ok, __eprintf calls __fprintfieee128 rather than fprintf from the other gcc, but maybe it is ok because it passes to fprintf only arguments that are not floating point. __fixtfdi calls __lekf2, so looks ABI incompatible, ditto __fixunstfdi, __floatditf calls __gcc_q{mul,add}, so might be ok, __fixtfti calls __lekf2, so looks ABI incompatible, ditto __fixunstfti, __floattitf also looks broken, ditto __floatuntitf. Ignoring the decimal stuff (_dpd*). So, if we want backwards ABI compatibility, I'm afraid when building libgcc, at least the *tf* entry points, they should be built with explicit -mabi=ibmlongdouble or otherwise ensure it is using IFmode rather than TFmode stuff.