cmd/compile: document regalloc fields

Document what the fields of regalloc mean.
Hopefully will help people understand how the register allocator works.

Change-Id: Ic322ed2019cc839b812740afe8cd2cf0b61da046
Reviewed-on: https://go-review.googlesource.com/137016
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/gen/main.go

@@ -63,9 +63,14 @@ type blockData struct {
 }
 
 type regInfo struct {
-	inputs   []regMask
+	// inputs[i] encodes the set of registers allowed for the i'th input.
+	// Inputs that don't use registers (flags, memory, etc.) should be 0.
+	inputs []regMask
+	// clobbers encodes the set of registers that are overwritten by
+	// the instruction (other than the output registers).
 	clobbers regMask
-	outputs  []regMask
+	// outpus[i] encodes the set of registers allowed for the i'th output.
+	outputs []regMask
 }
 
 type regMask uint64

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

@@ -50,9 +50,17 @@ type outputInfo struct {
 }
 
 type regInfo struct {
-	inputs   []inputInfo // ordered in register allocation order
+	// inputs encodes the register restrictions for an instruction's inputs.
+	// Each entry specifies an allowed register set for a particular input.
+	// They are listed in the order in which regalloc should pick a register
+	// from the register set (most constrained first).
+	// Inputs which do not need registers are not listed.
+	inputs []inputInfo
+	// clobbers encodes the set of registers that are overwritten by
+	// the instruction (other than the output registers).
 	clobbers regMask
-	outputs  []outputInfo // ordered in register allocation order
+	// outputs is the same as inputs, but for the outputs of the instruction.
+	outputs []outputInfo
 }
 
 type auxType int8

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

@@ -150,6 +150,8 @@ type register uint8
 
 const noRegister register = 255
 
+// A regMask encodes a set of machine registers.
+// TODO: regMask -> regSet?
 type regMask uint64
 
 func (m regMask) String() string {