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_fqMulLeg folded a negative c0 = a0*b0 - a1*b1 with
SHLQ $1,R10 / BTRQ $63,R9 / ADCQ R10,R8 / ADCQ R14,R9
SHLQ's carry-out is overwritten by the following BTRQ, so the top bit is
lost. R14, the sign extension of the borrow, is added plain instead of
double. Finally the carry out is dropped. These three errors cancel most
of the time, but not always.
Fix and add regression tests.
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cjpatton
approved these changes
Jul 31, 2026
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_fqMulLeg folded a negative c0 = a0b0 - a1b1 with
SHLQ's carry-out is overwritten by the following BTRQ, so the top bit is lost. R14, the sign extension of the borrow, is added plain instead of double. Finally the carry out is dropped. These three errors cancel most of the time, but not always.
Fix and add regression tests.