This is a variation on
https://github.com/gcc-mirror/gcc/blob/2f2aeda98f3aa24034a700e7efcb6c1a9397836f/gcc/testsuite/gcc.c-torture/execute/ieee/rbug.c
----
package main
/*
float s(unsigned long long k) {
float x;
x = (float)k;
return x;
}
unsigned long long cmain() {
unsigned long long int k;
double x;
k = 0x8234508000000001ULL;
x = s(k);
k = (unsigned long long)x;
return k;
}
*/
import "C"
import (
"fmt"
"runtime"
)
func s(k uint64) float32 {
var x float32
x = float32(k)
return x
}
func main() {
ck := uint64(C.cmain())
k := uint64(0x8234508000000001)
x := float64(s(k))
k = uint64(x)
fmt.Printf("%s, %s/%s\tC: %#x Go: %#x, equal: %v\n",
runtime.Version(), runtime.GOOS, runtime.GOARCH, ck, k, ck == k)
}
----
AFAICT, the values involved in the conversions are not out of range for
float32 or uint64. That's why I would expect the Go results to be the same
on any supported target. That's not the case, see linux/{386,arm}:
----
go1.16.2, openbsd/amd64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, darwin/amd64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, freebsd/amd64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, linux/386 C: 0x8234500000000000 Go: 0x8234500000000000,
equal: true
go1.17.1, linux/amd64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, linux/arm C: 0x8234510000000000 Go: 0x8234500000000000,
equal: false
go1.17.1, linux/arm64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, linux/s390x C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, netbsd/amd64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
go1.17.1, windows/amd64 C: 0x8234510000000000 Go: 0x8234510000000000,
equal: true
----
[Q1] Is that actually permitted by the Go specification? (If so I failed to
find that.)
Note that the results agree with C on all targets I have easy access to -
except linux/arm.
[Q2] Is that expected or not? I'm aware that the Go specification is silent
about such compatibility with C wrt rounding/conversions.
Thanks in advance for any insights.
-j
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