Question

some program in C which does extensive floating point calculations get right results on a pc linux box, but wrong results on the SPE of the cell processor, but not on the PPU of the cell. I am using gcc compilers. I wonder if there is some gcc compilation option to increase rounding method or similar so I get single float precision calculations with more precision. I can not change to double, as on the SPE performance would drastic reduce

Thanks

Answer 1

Based on the IBM documentation for the differences from IEEE 754 on the SPU, it could be any number of things:

• Zero results from arithmetic operations are always +0, never -0.
• Denormal inputs from 2-149 to 2-126 to arithmetic operations are treated
  as zero with the same sign. Arithmetic
  operations never produce denormal
  results, but produce +0 instead.
• Arithmetic operations do not support IEEE Inf or NaN. These bit patterns
  represent valid numbers. Overflow
  results produce the maximum magnitude
  value of appropriate sign.
• Arithmetic operations use only the round-to-zero (chop, truncate)
  rounding mode, regardless of the
  setting of the rounding mode in the
  Floating-Point Status and Control
  Register (FPSCR), which affects only
  double-precision arithmetic
  operations.

Of course, on a related page, you can also compile SPU code for strict IEEE conformance:

By default, XL C/C++ follows most, but
not all of the rules in the IEEE
standard. If you compile with the
-qnostrict option, which is enabled by default at optimization level -O3 or
higher, some IEEE floating-point rules
are violated in ways that can improve
performance but might affect program
correctness. To avoid this issue, and
to compile for strict compliance with
the IEEE standard, do the following:

• Use the -qfloat=nomaf compiler option.
• If the program changes the rounding mode at runtime, use the -qfloat=rrm
  option.
• If the data or program code contains signaling NaN values (NaNS), use the
  -qfloat=nans option. (A signaling NaN is different from a quiet NaN; you
  must explicitly code it into the
  program or data or create it by using
  the -qinitauto compiler option.)
• If you compile with -O3, -O4, or -O5, include the option -qstrict after it.