Revert "runtime: adjust softfloat corner cases to match amd64/arm64"

This reverts commit b9f3accdcf973ca41069e22e6859b9436801aae5.

Reason for revert: we need to do this more carefully, at minimum gated by a module version

(This should follow the WASM FP conversion revert)

Change-Id: Ib98ce7d243348f69c9944db8537397b225c2cc33
Reviewed-on: https://go-review.googlesource.com/c/go/+/711841
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Bypass: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>

1 files changed, 23 insertions, 70 deletions

diff --git a/src/runtime/softfloat64.go b/src/runtime/softfloat64.go
index 7b9409f75b..42ef009297 100644
--- a/src/runtime/softfloat64.go
+++ b/src/runtime/softfloat64.go
@@ -26,11 +26,6 @@ const (
 	neg32 uint32 = 1 << (expbits32 + mantbits32)
 )
 
-// If F is not NaN and not Inf, then f == (-1)**sign * mantissa * 2**(exp-52)
-// The mantissa and exp are adjusted from their stored representation  so
-// that the mantissa includes the formerly implicit 1, the exponent bias
-// is removed, and denormalized floats to put a 1 in the expected
-// (1<<mantbits64) position.
 func funpack64(f uint64) (sign, mant uint64, exp int, inf, nan bool) {
 	sign = f & (1 << (mantbits64 + expbits64))
 	mant = f & (1<<mantbits64 - 1)
@@ -376,25 +371,24 @@ func fcmp64(f, g uint64) (cmp int32, isnan bool) {
 	return 0, false
 }
 
-// returns saturated-conversion int64 value of f
-// and whether the input was NaN (in which case it
-// may not match the "hardware" conversion).
-func f64toint(f uint64) (val int64, isNan bool) {
+func f64toint(f uint64) (val int64, ok bool) {
 	fs, fm, fe, fi, fn := funpack64(f)
 
 	switch {
-
-	case fn: // NaN
-		return -0x8000_0000_0000_0000, false
+	case fi, fn: // NaN
+		return 0, false
 
 	case fe < -1: // f < 0.5
 		return 0, false
 
-	case fi || fe >= 63: // |f| >= 2^63, including infinity
+	case fe > 63: // f >= 2^63
+		if fs != 0 && fm == 0 { // f == -2^63
+			return -1 << 63, true
+		}
 		if fs != 0 {
-			return -0x8000_0000_0000_0000, true
+			return 0, false
 		}
-		return 0x7fff_ffff_ffff_ffff, true
+		return 0, false
 	}
 
 	for fe > int(mantbits64) {
@@ -406,51 +400,12 @@ func f64toint(f uint64) (val int64, isNan bool) {
 		fm >>= 1
 	}
 	val = int64(fm)
-	if val < 0 {
-		if fs != 0 {
-			return -0x8000_0000_0000_0000, true
-		}
-		return 0x7fff_ffff_ffff_ffff, true
-	}
 	if fs != 0 {
 		val = -val
 	}
 	return val, true
 }
 
-// returns saturated-conversion uint64 value of f
-// and whether the input was NaN (in which case it
-// may not match the "hardware" conversion).
-func f64touint(f uint64) (val uint64, isNan bool) {
-	fs, fm, fe, fi, fn := funpack64(f)
-
-	switch {
-
-	case fn: // NaN
-		return 0xffff_ffff_ffff_ffff, false
-
-	case fs != 0: // all negative, including -Inf, are zero
-		return 0, true
-
-	case fi || fe >= 64: // positive infinity or f >= 2^64
-		return 0xffff_ffff_ffff_ffff, true
-
-	case fe < -1: // f < 0.5
-		return 0, true
-	}
-
-	for fe > int(mantbits64) {
-		fe--
-		fm <<= 1
-	}
-	for fe < int(mantbits64) {
-		fe++
-		fm >>= 1
-	}
-	val = fm
-	return val, true
-}
-
 func fintto64(val int64) (f uint64) {
 	fs := uint64(val) & (1 << 63)
 	mant := uint64(val)
@@ -609,12 +564,6 @@ func fint64to64(x int64) uint64 {
 
 func f32toint32(x uint32) int32 {
 	val, _ := f64toint(f32to64(x))
-	if val >= 0x7fffffff {
-		return 0x7fffffff
-	}
-	if val < -0x80000000 {
-		return -0x80000000
-	}
 	return int32(val)
 }
 
@@ -625,12 +574,6 @@ func f32toint64(x uint32) int64 {
 
 func f64toint32(x uint64) int32 {
 	val, _ := f64toint(x)
-	if val >= 0x7fffffff {
-		return 0x7fffffff
-	}
-	if val < -0x80000000 {
-		return -0x80000000
-	}
 	return int32(val)
 }
 
@@ -640,13 +583,23 @@ func f64toint64(x uint64) int64 {
 }
 
 func f64touint64(x uint64) uint64 {
-	val, _ := f64touint(x)
-	return val
+	var m uint64 = 0x43e0000000000000 // float64 1<<63
+	if fgt64(m, x) {
+		return uint64(f64toint64(x))
+	}
+	y := fadd64(x, -m)
+	z := uint64(f64toint64(y))
+	return z | (1 << 63)
 }
 
 func f32touint64(x uint32) uint64 {
-	val, _ := f64touint(f32to64(x))
-	return val
+	var m uint32 = 0x5f000000 // float32 1<<63
+	if fgt32(m, x) {
+		return uint64(f32toint64(x))
+	}
+	y := fadd32(x, -m)
+	z := uint64(f32toint64(y))
+	return z | (1 << 63)
 }
 
 func fuint64to64(x uint64) uint64 {