runtime: use correct truncated constants for float conversion

There is a range of numbers lower than 0x7fff_ffff_ffff_ffff which
cannot be represented by a 64 bit float. We set that to the correct
limit beyond which conversions can happen properly.

It appears that the negative bound check can indeed by correctly handled
by I64TruncF64S. But we use the same limit for consistency.

Fixes #38839

Change-Id: Ib783a22cb331fba7e6955459f41c67f9ceb53461
Reviewed-on: https://go-review.googlesource.com/c/go/+/231837
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 131 insertions, 3 deletions

diff --git a/src/runtime/conv_wasm_test.go b/src/runtime/conv_wasm_test.go
new file mode 100644
index 0000000000..5054fca04d
--- /dev/null
+++ b/src/runtime/conv_wasm_test.go
@@ -0,0 +1,128 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"testing"
+)
+
+var res int64
+var ures uint64
+
+func TestFloatTruncation(t *testing.T) {
+	testdata := []struct {
+		input      float64
+		convInt64  int64
+		convUInt64 uint64
+		overflow   bool
+	}{
+		// max +- 1
+		{
+			input:      0x7fffffffffffffff,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		// For out-of-bounds conversion, the result is implementation-dependent.
+		// This test verifies the implementation of wasm architecture.
+		{
+			input:      0x8000000000000000,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      0x7ffffffffffffffe,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		// neg max +- 1
+		{
+			input:      -0x8000000000000000,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      -0x8000000000000001,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      -0x7fffffffffffffff,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		// trunc point +- 1
+		{
+			input:      0x7ffffffffffffdff,
+			convInt64:  0x7ffffffffffffc00,
+			convUInt64: 0x7ffffffffffffc00,
+		},
+		{
+			input:      0x7ffffffffffffe00,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      0x7ffffffffffffdfe,
+			convInt64:  0x7ffffffffffffc00,
+			convUInt64: 0x7ffffffffffffc00,
+		},
+		// neg trunc point +- 1
+		{
+			input:      -0x7ffffffffffffdff,
+			convInt64:  -0x7ffffffffffffc00,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      -0x7ffffffffffffe00,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      -0x7ffffffffffffdfe,
+			convInt64:  -0x7ffffffffffffc00,
+			convUInt64: 0x8000000000000000,
+		},
+		// umax +- 1
+		{
+			input:      0xffffffffffffffff,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      0x10000000000000000,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      0xfffffffffffffffe,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		// umax trunc +- 1
+		{
+			input:      0xfffffffffffffbff,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0xfffffffffffff800,
+		},
+		{
+			input:      0xfffffffffffffc00,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0x8000000000000000,
+		},
+		{
+			input:      0xfffffffffffffbfe,
+			convInt64:  -0x8000000000000000,
+			convUInt64: 0xfffffffffffff800,
+		},
+	}
+	for _, item := range testdata {
+		if got, want := int64(item.input), item.convInt64; got != want {
+			t.Errorf("int64(%f): got %x, want %x", item.input, got, want)
+		}
+		if got, want := uint64(item.input), item.convUInt64; got != want {
+			t.Errorf("uint64(%f): got %x, want %x", item.input, got, want)
+		}
+	}
+}
diff --git a/src/runtime/sys_wasm.s b/src/runtime/sys_wasm.s
index 41260bdf23..e7a6570095 100644
--- a/src/runtime/sys_wasm.s
+++ b/src/runtime/sys_wasm.s
@@ -99,7 +99,7 @@ TEXT runtime·wasmTruncS(SB), NOSPLIT, $0-0
 	End
 
 	Get R0
-	F64Const $9223372036854775807.
+	F64Const $0x7ffffffffffffc00p0 // Maximum truncated representation of 0x7fffffffffffffff
 	F64Gt
 	If
 		I64Const $0x8000000000000000
@@ -107,7 +107,7 @@ TEXT runtime·wasmTruncS(SB), NOSPLIT, $0-0
 	End
 
 	Get R0
-	F64Const $-9223372036854775808.
+	F64Const $-0x7ffffffffffffc00p0 // Minimum truncated representation of -0x8000000000000000
 	F64Lt
 	If
 		I64Const $0x8000000000000000
@@ -128,7 +128,7 @@ TEXT runtime·wasmTruncU(SB), NOSPLIT, $0-0
 	End
 
 	Get R0
-	F64Const $18446744073709551615.
+	F64Const $0xfffffffffffff800p0 // Maximum truncated representation of 0xffffffffffffffff
 	F64Gt
 	If
 		I64Const $0x8000000000000000