cmd/compile: hangs converting int const to complex64

[dvyukov]

gc hangs compiling the following program:

package p
func f() { _ = complex64(0x9EEbcab0Aa2DDe942Ea0aef5Aeb6bFF2dfcaf) }

go version devel +71832ff Fri Jul 3 21:39:04 2015 +0000 linux/amd64

[ALTree]

The compiler gets stuck trying to print an error message.

While compiling the above snippet, gc reaches the following line in the overflow function inside gc/compile/const.go:

Yyerror("constant %s overflows %v", Vconv(v, 0), t)

Vconv calls the Fconv function in gc/compile/mparith3, where gc dies in an infinite loop caused by the fact that the number to be printed in the error message is +Inf, but Fconv tries to scale it to a reasonable size in a loop before printing the error, dividing it by 10 and checking for m => 10 (which will never became false if m is +Inf).

A quick way to fix this is to change the overflow function so that it doesn't print any error when it gets Inf floats (which is correct, since +Inf does not overflows float32 o float64: it's a valid float32 value).

A const xxxx overflows float32 is already printed by the frontend when the number is parsed the first time, so we shouldn't loose any informative error message (I hope).

[ALTree]

Nope, looks like we lose the error message in certain cases. It's probably better to fix the Fconv function so that at least it prints something on Inf input.

[gopherbot]

CL https://golang.org/cl/16620 mentions this issue.

[rsc]

Dup of #11590.