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Diffstat (limited to 'src/passes/I64ToI32Lowering.cpp')
| -rw-r--r-- | src/passes/I64ToI32Lowering.cpp | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/src/passes/I64ToI32Lowering.cpp b/src/passes/I64ToI32Lowering.cpp index 78f15fcad..dfd0878f8 100644 --- a/src/passes/I64ToI32Lowering.cpp +++ b/src/passes/I64ToI32Lowering.cpp @@ -696,6 +696,38 @@ struct I64ToI32Lowering : public WalkerPass<PostWalker<I64ToI32Lowering>> { // Mostly just shuffling things around here with coercions and whatnot! // Note though that all arithmetic is done with f64 to have as much // precision as we can. + // + // NB: this is *not* accurate for i64 -> f32. Using f64s for intermediate + // operations can give slightly inaccurate results in some cases, as we + // round to an f64, then round again to an f32, which is not always the + // same as a single rounding of i64 to f32 directly. Example: + // + // #include <stdio.h> + // int main() { + // unsigned long long x = 18446743523953737727ULL; + // float y = x; + // double z = x; + // float w = z; + // printf("i64 : %llu\n" + // "i64->f32 : %f\n" + // "i64->f64 : %f\n" + // "i64->f64->f32: %f\n", x, y, z, w); + // } + // + // i64 : 18446743523953737727 + // i64->f32 : 18446742974197923840.000000 ;; correct rounding to f32 + // i64->f64 : 18446743523953737728.000000 ;; correct rounding to f64 + // i64->f64->f32: 18446744073709551616.000000 ;; incorrect rounding to f32 + // + // This is even a problem if we use BigInts in JavaScript to represent + // i64s, as Math.fround(BigInt) is not supported - the BigInt must be + // converted to a Number first, so we again have that extra rounding. + // + // A more precise approach could use compiled floatdisf/floatundisf from + // compiler-rt, but that is much larger and slower. (Note that we are in the + // interesting situation of having f32 and f64 operations and only missing + // i64 ones, so we have a different problem to solve than compiler-rt, and + // maybe there is a better solution we haven't found yet.) TempVar highBits = fetchOutParam(curr->value); TempVar lowBits = getTemp(); TempVar highResult = getTemp(); |