cmd/compile/internal/ssa: don't spill register offsets on amd64

Transform (ADDQconst SP) into (LEA SP), because lea is rematerializeable,
so this avoids register spill. We can't mark ADDQconst as rematerializeable,
because it clobbers flags. This makes go binary ~2kb smaller.

For reference here is generated code for function from bug report.
Before:
        CALL    "".g(SB)
        MOVBLZX (SP), AX
        LEAQ    8(SP), DI
        TESTB   AX, AX
        JEQ     15
        MOVQ    "".p(SP), SI
        DUFFCOPY        $196
        MOVQ    $0, (SP)
        PCDATA  $0, $1
        CALL    "".h(SB)
        RET
        MOVQ    DI, ""..autotmp_2-8(SP) // extra spill
        PCDATA  $0, $2
        CALL    "".g(SB)
        MOVQ    ""..autotmp_2-8(SP), DI // extra register fill
        MOVQ    "".p(SP), SI
        DUFFCOPY        $196
        MOVQ    $1, (SP)
        PCDATA  $0, $1
        CALL    "".h(SB)
        JMP     14
        END

After:
        CALL    "".g(SB)
        MOVBLZX (SP), AX
        TESTB   AX, AX
        JEQ     15
        LEAQ    8(SP), DI
        MOVQ    "".p(SP), SI
        DUFFCOPY        $196
        MOVQ    $0, (SP)
        PCDATA  $0, $1
        CALL    "".h(SB)
        RET
        PCDATA  $0, $0  // no spill
        CALL    "".g(SB)
        LEAQ    8(SP), DI // rematerialized instead
        MOVQ    "".p(SP), SI
        DUFFCOPY        $196
        MOVQ    $1, (SP)
        PCDATA  $0, $1
        CALL    "".h(SB)
        JMP     14
        END

Fixes #22947

Change-Id: I8f33b860dc6c8828373477171b172ca2ce30074f
Reviewed-on: https://go-review.googlesource.com/81815
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index db7c1a447b247e05b6498adca71a5c060f0c9ba8..e43b61b7c72946d11bca2bb1869b911c645129d2 100644 (file)
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/src/cmd/compile/internal/ssa/gen/AMD64.rules
@@ -2662,3 +2662,7 @@
 (MOVLf2i <t> (Arg [off] {sym})) -> @b.Func.Entry (Arg <t> [off] {sym})
 (MOVQi2f <t> (Arg [off] {sym})) -> @b.Func.Entry (Arg <t> [off] {sym})
 (MOVLi2f <t> (Arg [off] {sym})) -> @b.Func.Entry (Arg <t> [off] {sym})
+
+// LEAQ is rematerializeable, so this helps to avoid register spill.
+// See isuue 22947 for details
+(ADDQconst [off] x:(SP)) -> (LEAQ [off] x)

diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index 0b2b321d2566603215b22e78ca676f0820844737..be850041900c61c1d6f87fbf5a0776e26aa70ddf 100644 (file)
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -2054,6 +2054,20 @@ func rewriteValueAMD64_OpAMD64ADDQconst_0(v *Value) bool {
                v.AddArg(x)
                return true
        }
+       // match: (ADDQconst [off] x:(SP))
+       // cond:
+       // result: (LEAQ [off] x)
+       for {
+               off := v.AuxInt
+               x := v.Args[0]
+               if x.Op != OpSP {
+                       break
+               }
+               v.reset(OpAMD64LEAQ)
+               v.AuxInt = off
+               v.AddArg(x)
+               return true
+       }
        return false
 }
 func rewriteValueAMD64_OpAMD64ADDQconstmem_0(v *Value) bool {