cmd/compile: fix constant folding of right shifts on s390x

Repeat previous fix on amd64 for s390x.
Sub-word right shifts should sign extend before shifting.

Update #23812

Change-Id: I2d770190c7d8a22310b0dbd9facb3fb05afa362a
Reviewed-on: https://go-review.googlesource.com/94028
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/gen/S390X.rules

@@ -1091,7 +1091,7 @@
 (SUBconst (MOVDconst [d]) [c]) -> (MOVDconst [d-c])
 (SUBconst (SUBconst x [d]) [c]) && is32Bit(-c-d) -> (ADDconst [-c-d] x)
 (SRADconst [c] (MOVDconst [d])) -> (MOVDconst [d>>uint64(c)])
-(SRAWconst [c] (MOVDconst [d])) -> (MOVDconst [d>>uint64(c)])
+(SRAWconst [c] (MOVDconst [d])) -> (MOVDconst [int64(int32(d))>>uint64(c)])
 (NEG (MOVDconst [c])) -> (MOVDconst [-c])
 (NEGW (MOVDconst [c])) -> (MOVDconst [int64(int32(-c))])
 (MULLDconst [c] (MOVDconst [d])) -> (MOVDconst [c*d])

src/cmd/compile/internal/ssa/gen/S390XOps.go

@@ -310,15 +310,15 @@ func init() {
 		{name: "SLWconst", argLength: 1, reg: gp11, asm: "SLW", aux: "Int8"}, // arg0 << auxint, shift amount 0-31
 
 		{name: "SRD", argLength: 2, reg: sh21, asm: "SRD"},                   // unsigned arg0 >> arg1, shift amount is mod 64
-		{name: "SRW", argLength: 2, reg: sh21, asm: "SRW"},                   // unsigned arg0 >> arg1, shift amount is mod 32
+		{name: "SRW", argLength: 2, reg: sh21, asm: "SRW"},                   // unsigned uint32(arg0) >> arg1, shift amount is mod 32
 		{name: "SRDconst", argLength: 1, reg: gp11, asm: "SRD", aux: "Int8"}, // unsigned arg0 >> auxint, shift amount 0-63
-		{name: "SRWconst", argLength: 1, reg: gp11, asm: "SRW", aux: "Int8"}, // unsigned arg0 >> auxint, shift amount 0-31
+		{name: "SRWconst", argLength: 1, reg: gp11, asm: "SRW", aux: "Int8"}, // unsigned uint32(arg0) >> auxint, shift amount 0-31
 
 		// Arithmetic shifts clobber flags.
 		{name: "SRAD", argLength: 2, reg: sh21, asm: "SRAD", clobberFlags: true},                   // signed arg0 >> arg1, shift amount is mod 64
-		{name: "SRAW", argLength: 2, reg: sh21, asm: "SRAW", clobberFlags: true},                   // signed arg0 >> arg1, shift amount is mod 32
+		{name: "SRAW", argLength: 2, reg: sh21, asm: "SRAW", clobberFlags: true},                   // signed int32(arg0) >> arg1, shift amount is mod 32
 		{name: "SRADconst", argLength: 1, reg: gp11, asm: "SRAD", aux: "Int8", clobberFlags: true}, // signed arg0 >> auxint, shift amount 0-63
-		{name: "SRAWconst", argLength: 1, reg: gp11, asm: "SRAW", aux: "Int8", clobberFlags: true}, // signed arg0 >> auxint, shift amount 0-31
+		{name: "SRAWconst", argLength: 1, reg: gp11, asm: "SRAW", aux: "Int8", clobberFlags: true}, // signed int32(arg0) >> auxint, shift amount 0-31
 
 		{name: "RLLGconst", argLength: 1, reg: gp11, asm: "RLLG", aux: "Int8"}, // arg0 rotate left auxint, rotate amount 0-63
 		{name: "RLLconst", argLength: 1, reg: gp11, asm: "RLL", aux: "Int8"},   // arg0 rotate left auxint, rotate amount 0-31

src/cmd/compile/internal/ssa/rewriteS390X.go

@@ -37199,7 +37199,7 @@ func rewriteValueS390X_OpS390XSRAW_0(v *Value) bool {
 func rewriteValueS390X_OpS390XSRAWconst_0(v *Value) bool {
 	// match: (SRAWconst [c] (MOVDconst [d]))
 	// cond:
-	// result: (MOVDconst [d>>uint64(c)])
+	// result: (MOVDconst [int64(int32(d))>>uint64(c)])
 	for {
 		c := v.AuxInt
 		v_0 := v.Args[0]
@@ -37208,7 +37208,7 @@ func rewriteValueS390X_OpS390XSRAWconst_0(v *Value) bool {
 		}
 		d := v_0.AuxInt
 		v.reset(OpS390XMOVDconst)
-		v.AuxInt = d >> uint64(c)
+		v.AuxInt = int64(int32(d)) >> uint64(c)
 		return true
 	}
 	return false