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cmd/internal/gc: use 512 bits (rather than 464) for multi-precision arithmetic
The original implementation used 16 int "words" but only 29 bits per word for a total of 16*29 = 464 bits, with a space consumption of 16*64 = 1024 bits on a 64 bit machine. Switching to 512 bits increases precision while still using (in the worst case) half the amount of memory per mp value on a 64 bit machine. Also: Decreased permitted number of least-significant mantissa bits which may be incorrect when considering if a precise floating-point constant is an integer from 29 to 16 bits. Change-Id: Iee9287056f0e9aa4f06ceac0724ff4674f710c53 Reviewed-on: https://go-review.googlesource.com/8429 Reviewed-by: Russ Cox <rsc@golang.org>
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@ -57,10 +57,11 @@ const (
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)
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const (
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// TODO(gri) consider increasing Mpprec to 512 or perhaps 1024
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// (this would permit enabling additional tests).
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Mpprec = 16 * 29 // == 464, to match original value
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Mpdebug = 0
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// Maximum size in bits for Mpints before signalling
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// overflow and also mantissa precision for Mpflts.
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Mpprec = 512
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// Turn on for constant arithmetic debugging output.
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Mpdebug = false
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)
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// Mpint represents an integer constant.
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@ -35,10 +35,7 @@ func mpmovefltfix(a *Mpint, b *Mpflt) int {
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return 0
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}
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// TODO(gri) reduce the value of delta - currently
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// we use the size of a mp-word of the old implementation
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// for approximately similar behavior.
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const delta = 29 // a reasonably small number of bits > 0
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const delta = 16 // a reasonably small number of bits > 0
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var t big.Float
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t.SetPrec(Mpprec - delta)
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@ -32,13 +32,13 @@ func mpmovefltflt(a *Mpflt, b *Mpflt) {
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}
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func mpaddfltflt(a *Mpflt, b *Mpflt) {
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf("\n%v + %v", Fconv(a, 0), Fconv(b, 0))
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}
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a.Val.Add(&a.Val, &b.Val)
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf(" = %v\n\n", Fconv(a, 0))
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}
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}
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@ -51,25 +51,25 @@ func mpaddcflt(a *Mpflt, c float64) {
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}
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func mpsubfltflt(a *Mpflt, b *Mpflt) {
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf("\n%v - %v", Fconv(a, 0), Fconv(b, 0))
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}
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a.Val.Sub(&a.Val, &b.Val)
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf(" = %v\n\n", Fconv(a, 0))
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}
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}
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func mpmulfltflt(a *Mpflt, b *Mpflt) {
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf("%v\n * %v\n", Fconv(a, 0), Fconv(b, 0))
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}
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a.Val.Mul(&a.Val, &b.Val)
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf(" = %v\n\n", Fconv(a, 0))
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}
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}
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@ -82,13 +82,13 @@ func mpmulcflt(a *Mpflt, c float64) {
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}
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func mpdivfltflt(a *Mpflt, b *Mpflt) {
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf("%v\n / %v\n", Fconv(a, 0), Fconv(b, 0))
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}
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a.Val.Quo(&a.Val, &b.Val)
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf(" = %v\n\n", Fconv(a, 0))
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}
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}
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@ -140,13 +140,13 @@ func mpgetflt32(a *Mpflt) float64 {
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}
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func Mpmovecflt(a *Mpflt, c float64) {
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf("\nconst %g", c)
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}
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a.Val.SetFloat64(c)
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if Mpdebug != 0 {
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if Mpdebug {
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fmt.Printf(" = %v\n", Fconv(a, 0))
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}
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}
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@ -99,5 +99,13 @@ const (
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f88 = f87 * 88
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f89 = f88 * 89
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f90 = f89 * 90
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f91 = f90 * 91 // ERROR "overflow"
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f91 = f90 * 91
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f92 = f91 * 92
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f93 = f92 * 93
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f94 = f93 * 94
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f95 = f94 * 95
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f96 = f95 * 96
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f97 = f96 * 97
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f98 = f97 * 98
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f99 = f98 * 99 // ERROR "overflow"
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)
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@ -21,7 +21,7 @@ func main() {
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var prec float64
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switch runtime.Compiler {
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case "gc":
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prec = 16 * 29
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prec = 512
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case "gccgo":
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prec = 256
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default:
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