Fix float64 to uint{8,16,32} conversion. #1305
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Before this change, GopherJS compiler emitted `(f >> 0)` expression to convert a float64 `f` to any non-64-bit unsigned integer type. This is incorrect, because `>>` is a signed bitshift operator in JS, so the returned value remained signed. Moreover, it did not take into account to bit size of the target type. By removing the switch cause, we make the compiler fall through to the default clause where `fc.fixNumber()` actually does the right thing, taking the target into account. Fixes gopherjs#733.
flimzy
approved these changes
May 6, 2024
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Before this change, GopherJS compiler emitted
(f >> 0)expression to convert a float64fto any non-64-bit unsigned integer type. This is incorrect, because>>is a signed bitshift operator in JS, so the returned value remained signed. Moreover, it did not take into account to bit size of the target type.By removing the switch cause, we make the compiler fall through to the default clause where
fc.fixNumber()actually does the right thing, taking the target into account.Fixes #733.
Amazingly, this is what was causing flakiness of the
hash/maphash.TestHashHighBytessince Go 1.19. There, the 64-bit hash value was computed as(uint64(hi) << 32) | uint64(lo), wherehiandlowere randomly-generatedunit32values. In turn, those values were generated by converting a random [0, 2^32) float64 value into uint32. As per explanation above, this used to be done with the following JS expression(f >> 0), which meant that for values in the range of [2^31, 2^32) it would return a negative JS number. This would also set all the high bits of that number to 1. So when we come back to(uint64(hi) << 32) | uint64(lo), there was the 50% chance that all higher bits of the right bitwise-or operand would be 1, meaning the result would also have the higher bits set to 1, regardless of whathiwas. That led to a high likelihood of failing said test 🕵️♂️