[dev.ssa] cmd/compile: implement GO386=387

Last part of the 386 SSA port.

Modify the x86 backend to simulate SSE registers and
instructions with 387 registers and instructions.
The simulation isn't terribly performant, but it works,
and the old implementation wasn't very performant either.
Leaving to people who care about 387 to optimize if they want.

Turn on SSA backend for 386 by default.

Fixes #16358

Change-Id: I678fb59132620b2c47e993c1c10c4c21135f70c0
Reviewed-on: https://go-review.googlesource.com/25271
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/src/runtime/vlrt.go b/src/runtime/vlrt.go
index cd37828ae4..7300f55dad 100644
--- a/src/runtime/vlrt.go
+++ b/src/runtime/vlrt.go
@@ -255,3 +255,17 @@ func slowdodiv(n, d uint64) (q, r uint64) {
 	}
 	return q, n
 }
+
+// Floating point control word values for GOARCH=386 GO386=387.
+// Bits 0-5 are bits to disable floating-point exceptions.
+// Bits 8-9 are the precision control:
+//   0 = single precision a.k.a. float32
+//   2 = double precision a.k.a. float64
+// Bits 10-11 are the rounding mode:
+//   0 = round to nearest (even on a tie)
+//   3 = round toward zero
+var (
+	controlWord64      uint16 = 0x3f + 2<<8 + 0<<10
+	controlWord32             = 0x3f + 0<<8 + 0<<10
+	controlWord64trunc        = 0x3f + 2<<8 + 3<<10
+)