PPC64: Poor StrictMath performance due to non-optimized compilation
gromero at linux.vnet.ibm.com
Tue Nov 22 00:34:37 UTC 2016
On 17-11-2016 19:48, Chris Plummer wrote:
>> The fdlibm code relies on aliasing a two-element array of int with a double to do bit-level reads and writes of floating-point values. As I understand it, the C spec allows compilers to assume
>> values of different types don't overlap in memory. The compilation environment has to be configured in such a way that the C compiler disables code generation and optimization techniques that would
>> run afoul of these fdlibm coding practices.
> This is the strict aliasing issue right? It's a long standing problem with fdlibm that kept getting worse as gcc got smarter. IIRC, compiling with -fno-strict-aliasing fixes it, but it's been more
> than 12 years since I last dealt with fdlibm and compiler aliasing issues.
I've tested with -O3 and -fno-strict-aliasing as you suggested but it did not
fix the fp precision issue on PPC64.
After finding that -fno-expensive-optimizations solved the problem, we narrowed
down the problem to the FMA: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78386
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