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>
diff --git a/src/cmd/compile/internal/ssa/gen/main.go b/src/cmd/compile/internal/ssa/gen/main.go
index f35a991..f7195bf 100644
--- a/src/cmd/compile/internal/ssa/gen/main.go
+++ b/src/cmd/compile/internal/ssa/gen/main.go
@@ -63,9 +63,14 @@
}
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
diff --git a/src/cmd/compile/internal/ssa/op.go b/src/cmd/compile/internal/ssa/op.go
index 6109218..43f5c59 100644
--- a/src/cmd/compile/internal/ssa/op.go
+++ b/src/cmd/compile/internal/ssa/op.go
@@ -50,9 +50,17 @@
}
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
diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index 278da6f..8946cf6 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -150,6 +150,8 @@
const noRegister register = 255
+// A regMask encodes a set of machine registers.
+// TODO: regMask -> regSet?
type regMask uint64
func (m regMask) String() string {
