[dev.ssa] cmd/compile: support NaCl in SSA for ARM

NaCl code runs in sandbox and there are restrictions for its
instruction uses
(https://developer.chrome.com/native-client/reference/sandbox_internals/arm-32-bit-sandbox).

Like the legacy backend, on NaCl,
- don't use R9, which is used as NaCl's "thread pointer".
- don't use Duff's device.
- don't use indexed load/stores.
- the assembler rewrites DIV/MOD to runtime calls, which on NaCl
  clobbers R12, so R12 is marked as clobbered for DIV/MOD.
- other restrictions are satisfied by the assembler.

Enable SSA specific tests on nacl/arm, and disable non-SSA ones.

Updates #15365.

Change-Id: I9262693ec6756b89ca29d3ae4e52a96fe5403b02
Reviewed-on: https://go-review.googlesource.com/24859
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/config.go

@@ -30,6 +30,7 @@ type Config struct {
 	ctxt            *obj.Link                  // Generic arch information
 	optimize        bool                       // Do optimization
 	noDuffDevice    bool                       // Don't use Duff's device
+	nacl            bool                       // GOOS=nacl
 	sparsePhiCutoff uint64                     // Sparse phi location algorithm used above this #blocks*#variables score
 	curFunc         *Func
 
@@ -175,13 +176,25 @@ func NewConfig(arch string, fe Frontend, ctxt *obj.Link, optimize bool) *Config
 	}
 	c.ctxt = ctxt
 	c.optimize = optimize
+	c.nacl = obj.Getgoos() == "nacl"
 
-	// Don't use Duff's device on Plan 9, because floating
+	// Don't use Duff's device on Plan 9 AMD64, because floating
 	// point operations are not allowed in note handler.
-	if obj.Getgoos() == "plan9" {
+	if obj.Getgoos() == "plan9" && arch == "amd64" {
 		c.noDuffDevice = true
 	}
 
+	if c.nacl {
+		c.noDuffDevice = true // Don't use Duff's device on NaCl
+
+		// ARM assembler rewrites DIV/MOD to runtime calls, which
+		// clobber R12 on nacl
+		opcodeTable[OpARMDIV].reg.clobbers |= 1 << 12  // R12
+		opcodeTable[OpARMDIVU].reg.clobbers |= 1 << 12 // R12
+		opcodeTable[OpARMMOD].reg.clobbers |= 1 << 12  // R12
+		opcodeTable[OpARMMODU].reg.clobbers |= 1 << 12 // R12
+	}
+
 	// Assign IDs to preallocated values/blocks.
 	for i := range c.values {
 		c.values[i].ID = ID(i)