Fixing undefined behaviours #4879
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`b_width_capped` already exists for preventing arithmetic overflow, limiting the value of `b_width` to 30. This just changes the left shifts to also use it. The caveat of incorrect results for extremely large values of `a_width` still applies, as does the improbability of that actually happening. This fixes #4844 (or at least, the floating point exception; the circuit still isn't valid but I think that's fine).
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@povik the |
log2(0) returns -inf, which gives undefined behaviour when casting to an int. So catch the case when it's 0 just set the width to 0.
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I don't quite understand that code so I don't feel like I can sign off on it, apologies. |
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@KrystalDelusion sure thing, and thanks for taking this up |
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What are the reasons/motivation for this change?
#4845 (and #4844 (comment)) reveal undefined behaviors in the Yosys code. This PR aims to resolve them.
Explain how this is achieved.
b_width_cappedalready exists for preventing arithmetic overflow, limiting the value ofb_widthto 30. This just changes the left shifts to also use it. The caveat of incorrect results for extremely large values ofa_widthstill applies, as does the improbability of that actually happening. This fixes synth failure #4844 (or at least, the floating point exception; the circuit still isn't valid but I think that's fine).In the
nowrshmskhandling,log2(0)returns -inf, which gives undefined behaviour when casting to an int. So catch the case when the stride is 0 and just set the stride_width to 0.If applicable, please suggest to reviewers how they can test the change.
Follow instructions for testing #4845 on this branch