interflop / interflop-backend-mcaint

interflop-backend-mcaint

Arguments

The MCA backends implement Montecarlo Arithmetic.

• libinterflop_mca_int.so: uses integer types to represent stochastic noise. In most architectures, this backend should be faster. The MCA integer backend only supports default precision and relative error mode; some user options are therefore unavailable.

VFC_BACKENDS="libinterflop_mca_int.so --help" ./test
test: verificarlo loaded backend libinterflop_mca_int.so
Usage: libinterflop_mca_int.so [OPTION...]

  -m, --mode=MODE            select MCA mode among {ieee, mca, pb, rr}
      --precision-binary32=PRECISION
                             select precision for binary32 (PRECISION >= 0)
      --precision-binary64=PRECISION
                             select precision for binary64 (PRECISION >= 0)
      --error-mode=ERR_MODE  select error mode among (rel, abs, all)
      --max-abs-error-exponent=ERR_EXPONENT
                             select the magnitude of the maximum allowed
                             absolute error (this option is only used when
                             error-mode={abs, all})
  -d, --daz                  denormals-are-zero: sets denormals inputs to zero
  -f, --ftz                  flush-to-zero: sets denormal output to zero
  -s, --seed=SEED            fix the random generator seed
  -?, --help                 Give this help list
      --usage                Give a short usage message

Two options control the behavior of the MCA backend.

The option --mode=MODE controls the arithmetic error mode. It accepts the following case insensitive values:

• mca: (default mode) Montecarlo Arithmetic with inbound and outbound errors
• ieee: the program uses standard IEEE arithmetic, no errors are introduced
• pb: Precision Bounding inbound errors only
• rr: Random Rounding outbound errors only

The option --precision-binary64=PRECISION controls the virtual precision used for the floating point operations in double precision (respectively for single precision with --precision-binary32). It accepts an integer value that represents the virtual precision at which MCA operations are performed. Its default value is 53 for binary64 and 24 for binary32. A precise definition of the virtual precision is given here.

One should note when using the QUAD backend, that the round operations during MCA computation always use round-to-zero mode.

In Random Round mode, the exact operations in given virtual precision are preserved.

The option --error-mode=ERR_MODE controls the way in which the error is interpreted. It accepts the following modes:

• rel: (default mode) the error is specified relative to the magnitude of the floating-point number
• abs: the error threshold is specified as an absolute value, independent of the value of the floating-point number, to be interpreted as 2^ERR_EXPONENT
• all: both relative and absolute modes are active simultaneously

The option --max-abs-error-exponent=ERR_EXPONENT is used only when the option --error-mode=ERR_MODE is active and controls the magnitude of the error threshold, when in absolute error mode or all mode. The error thershold is set to 2^ERR_EXPONENT.

The options --daz and --ftz flush subnormal numbers to 0. The --daz (Denormals-Are-Zero) flushes subnormal inputs to 0. The --ftz (Flush-To-Zero) flushes subnormal output to 0.

   $ VFC_BACKENDS="libinterflop_mca_int.so --mode=ieee" ./test
   0x0.fffffep-126 +0x1.000000p-149 = 0x1.000000p-126
   $ VFC_BACKENDS="libinterflop_mca_int.so --mode=ieee --daz" ./test
   0x0.fffffep-126 +0x1.000000p-149 = 0x0
   $ VFC_BACKENDS="libinterflop_mca_int.so --mode=ieee --ftz" ./test
   0x0.fffffep-126 +0x1.000000p-149 = 0x1.000000p-126

The option --seed fixes the random generator seed. It should not generally be used except if one to reproduce a particular MCA trace.