go/constant: avoid generating rats for large negative exponents

Fixes #20228

Change-Id: I1893ae3e192da01f9befe5469b2a32e534a691ba
Reviewed-on: https://go-review.googlesource.com/42592
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/go/constant/value.go b/src/go/constant/value.go
index e9b6087baead519657d0836e2cc581c1b85c343f..5474e73e24beadb285f1d07605a099ea540c70fe 100644 (file)
--- a/src/go/constant/value.go
+++ b/src/go/constant/value.go
@@ -247,6 +247,13 @@ func makeFloatFromLiteral(lit string) Value {
        if f, ok := newFloat().SetString(lit); ok {
                if smallRat(f) {
                        // ok to use rationals
+                       if f.Sign() == 0 {
+                               // Issue 20228: If the float underflowed to zero, parse just "0".
+                               // Otherwise, lit might contain a value with a large negative exponent,
+                               // such as -6e-1886451601. As a float, that will underflow to 0,
+                               // but it'll take forever to parse as a Rat.
+                               lit = "0"
+                       }
                        r, _ := newRat().SetString(lit)
                        return ratVal{r}
                }

diff --git a/src/go/constant/value_test.go b/src/go/constant/value_test.go
index 8a8a08eaaa18b82d14b4a5deef0d09ef27a953d8..954a0e05db35bdb38bc86757a388504c8ed21881 100644 (file)
--- a/src/go/constant/value_test.go
+++ b/src/go/constant/value_test.go
@@ -244,7 +244,8 @@ var stringTests = []struct {
        {"1e9999", "1e+9999", "0x.f8d4a9da224650a8cb2959e10d985ad92adbd44c62917e608b1f24c0e1b76b6f61edffeb15c135a4b601637315f7662f325f82325422b244286a07663c9415d2p+33216"},
        {"1e-9999", "1e-9999", "0x.83b01ba6d8c0425eec1b21e96f7742d63c2653ed0a024cf8a2f9686df578d7b07d7a83d84df6a2ec70a921d1f6cd5574893a7eda4d28ee719e13a5dce2700759p-33215"},
        {"2.71828182845904523536028747135266249775724709369995957496696763", "2.71828", "271828182845904523536028747135266249775724709369995957496696763/100000000000000000000000000000000000000000000000000000000000000"},
-       {"0e9999999999", "0", "0"}, // issue #16176
+       {"0e9999999999", "0", "0"},   // issue #16176
+       {"-6e-1886451601", "0", "0"}, // issue #20228
 
        // Complex
        {"0i", "(0 + 0i)", "(0 + 0i)"},