Fix sign extension for fractional power-of-two fixed-point products

[nindanaoto]

Summary

Fix fixed-point multiplication by positive fractional powers of two for negative operands.

SGen optimizes multiplication by constants such as 0.5 and 0.25 into shifts. For fractional constants, the generated RTL previously filled the high bits with zeros, effectively performing a logical right shift.

Because FixedPoint values use signed two's-complement representation, negative operands require an arithmetic right shift. The previous behavior could therefore turn a negative result into a large positive value.

Fix

Replicate the input sign bit when shifting right by a fractional power-of-two constant.

Positive operands retain the existing behavior, while negative operands now preserve their sign.

Tests

Add regression coverage for multiplication by:

• 0.5 with positive and negative operands
• 0.25 with positive and negative operands

The first commit adds the regression test and demonstrates the failure on the existing implementation. The second commit applies the fix and makes the test pass.

Tested with:

sbt "testOnly ir.rtl.hardwaretype.FixedPointTest"

Result:

Tests: succeeded 3, failed 0