cmd/compile: loads from readonly globals into const for mips64x

Ref: CL 141118
Update #26498

Change-Id: If4ea55c080b9aa10183eefe81fefbd4072deaf3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/280646
Trust: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Author: mengzhuo

src/cmd/compile/internal/ssa/gen/MIPS64.rules

@@ -681,3 +681,9 @@
 (GTZ (MOVVconst [c]) yes no) && c <= 0 => (First no yes)
 (GEZ (MOVVconst [c]) yes no) && c >= 0 => (First yes no)
 (GEZ (MOVVconst [c]) yes no) && c <  0 => (First no yes)
+
+// fold readonly sym load
+(MOVBload [off] {sym} (SB) _) && symIsRO(sym) => (MOVVconst [int64(read8(sym, int64(off)))])
+(MOVHload [off] {sym} (SB) _) && symIsRO(sym) => (MOVVconst [int64(read16(sym, int64(off), config.ctxt.Arch.ByteOrder))])
+(MOVWload [off] {sym} (SB) _) && symIsRO(sym) => (MOVVconst [int64(read32(sym, int64(off), config.ctxt.Arch.ByteOrder))])
+(MOVVload  [off] {sym} (SB) _) && symIsRO(sym) => (MOVVconst [int64(read64(sym, int64(off), config.ctxt.Arch.ByteOrder))])

src/cmd/compile/internal/ssa/rewriteMIPS64.go

@@ -30,6 +30,7 @@ func ConstantLoad() {
 	//   arm:`MOVW\t\$48`,`MOVW\t\$49`,`MOVW\t\$50`
 	// arm64:`MOVD\t\$12592`,`MOVD\t\$50`
 	//  wasm:`I64Const\t\$12592`,`I64Store16\t\$0`,`I64Const\t\$50`,`I64Store8\t\$2`
+	// mips64:`MOVV\t\$48`,`MOVV\t\$49`,`MOVV\t\$50`
 	bsink = []byte("012")
 
 	// 858927408 = 0x33323130