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cmd/compile: constant arithmetic with typed complex numbers is too precise #33285

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mdempsky opened this issue Jul 25, 2019 · 3 comments

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@mdempsky
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commented Jul 25, 2019

When computing constant arithmetic of typed floating-point numbers, we truncate the results based on the floating-point type. That is, if x and y are float32 constants, then x + y will be truncated to fit into a float32 too, even if we could compute it more precisely:

This is currently handled in const.go:setconst:

// Truncate precision for non-ideal float.
if v.Ctype() == CTFLT && n.Type.Etype != TIDEAL {
n.SetVal(Val{truncfltlit(v.U.(*Mpflt), n.Type)})
}

However, there's no corresponding logic for truncating typed complex constants. E.g., see this sample program that demonstrates "catastrophic cancellation" for arithmetic on const floats, non-const floats, and non-const complexes, but (non-spec-compliantly) computes the precise result for the const complex arithmetic: https://play.golang.org/p/UpXhGv8pLjQ

gccgo correctly computes 0 for all results. go/types looks like it probably handles this correctly too, but cmd/gotype doesn't easily expose constant values, and I haven't tried writing a test case.

Tagging "help wanted" since this is probably an easy CL if someone wants to grab it. There's already trunccmplxlit, just need to call it.

@mdempsky mdempsky added this to the Go1.14 milestone Jul 25, 2019

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commented Jul 26, 2019

Change https://golang.org/cl/187697 mentions this issue: cmd/compile: constant arithmetic with typed complex numbers

@bkpathak

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commented Jul 27, 2019

@mdempsky Hi Matthew, I can work on this.

@mdempsky

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commented Jul 27, 2019

@bkpathak Thanks for the offer! However, @choleraehyq has already started work on this. See CL 187697 linked above.

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