If -mfpu=vfp is enabled, the compiler will generate VFP instructions in the asm code directly rather than calls to the libgcc soft-float code. Even if the libgcc soft-float function could be replaced with the VFP instructions, you still get an additional branch to those operations and probably lower performance than complete VFP optimisation. Please note that I haven't tried this approach comment more on the performance.
The libm is a candidate for this optimisation but there are applications that would themselves benefit from being compiled with VFP. As I understand, there are difficulties in maintaining two separate variants for some packages (like OpenOffice). There are no patches to enable full VFP emulation. AFAIK, the Linux kernel community weren't keen to get such patches merged into mainline because of emulation performance reasons. Just for clarification, the kernel currently needs to trap the VFP exceptions for 3 reasons: 1. The VFP is disabled at a context switch and the first encounter of a VFP instruction triggers an undefined exception. At this point, the kernel saves the VFP registers for the old application and loads those for the new one. 2. On VFPv2 (found on ARMv5 and ARMv6 processors), the hardware does not implement full IEEE754 compliance. There are corner cases (like denormalised numbers) which aren't supported by hardware but the kernel traps and emulates them. Note that the VFPv3 (on ARMv7 processors) has full support for the IEEE754 compliance and there is no need for additional kernel emulation (though the code is still there since it's harmless). 3. Floating point operations exception (e.g. divide by zero) if the user application enabled them and the hardware supports them. Because of point 2 above, we have almost all the emulation code needed. The only missing part is the emulation of the VFP registers (the kernel currently reads the hardware ones) and maybe some optimisation to read ahead and emulate more than one instruction in the exception handler before returning to user. -- IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. -- armel gcc default optimisations https://bugs.launchpad.net/bugs/303232 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
