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Summary: Make sure we use SETGE instead of SETGT when checking if the sign bit is zero at SMULFIXSAT expansion. The faulty expansion occured when doing "expand" of SMULFIXSAT and the scale was exactly matching the size of the smaller type. For example doing i64 Z = SMULFIXSAT X, Y, 32 and expanding X/Y/Z into using two i32 values. The problem was that we sometimes did not saturate to min when overflowing. Here is an example using Q3.4 numbers: Consider that we are multiplying X and Y. X = 0x80 (-8.0 as Q3.4) Y = 0x20 (2.0 as Q3.4) To avoid loss of precision we do a widening multiplication, getting a 16 bit result Z = 0xF000 (-16.0 as Q7.8) To detect negative overflow we should check if the five most significant bits in Z are less than -1. Assume that we name the 4 most significant bits as HH and the next 4 bits as HL. Then we can do the check by examining if (HH < -1) or (HH == -1 && "sign bit in HL is zero"). The fault was that we have been doing the check as (HH < -1) or (HH == -1 && HL > 0) instead of (HH < -1) or (HH == -1 && HL >= 0). In our example HH is -1 and HL is 0, so the old code did not trigger saturation and simply truncated the result to 0x00 (0.0). With the bugfix we instead detect that we should saturate to min, and the result will be set to 0x80 (-8.0). Reviewers: leonardchan, bevinh Reviewed By: leonardchan Subscribers: hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D64331 llvm-svn: 365455
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