cmd/compile: don't generate panicshift for masked int shifts

We know that a & 31 is non-negative for all a, signed or not.
We can avoid checking that and needing to write out an
unreachable call to panicshift.

Change-Id: I32f32fb2c950d2b2b35ac5c0e99b7b2dbd47f917
Reviewed-on: https://go-review.googlesource.com/c/go/+/167499
Run-TryBot: Josh Bleecher Snyder <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: josharian

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

@@ -420,6 +420,11 @@
 (Less(64|32|16|8)    (Const(64|32|16|8) [c]) (Const(64|32|16|8) [d])) -> (ConstBool [b2i(c < d)])
 (Leq(64|32|16|8)     (Const(64|32|16|8) [c]) (Const(64|32|16|8) [d])) -> (ConstBool [b2i(c <= d)])
 
+(Geq8  (And8  _ (Const8  [c])) (Const8  [0])) && int8(c)  >= 0 -> (ConstBool [1])
+(Geq16 (And16 _ (Const16 [c])) (Const16 [0])) && int16(c) >= 0 -> (ConstBool [1])
+(Geq32 (And32 _ (Const32 [c])) (Const32 [0])) && int32(c) >= 0 -> (ConstBool [1])
+(Geq64 (And64 _ (Const64 [c])) (Const64 [0])) && int64(c) >= 0 -> (ConstBool [1])
+
 (Greater64U (Const64 [c]) (Const64 [d])) -> (ConstBool [b2i(uint64(c) > uint64(d))])
 (Greater32U (Const32 [c]) (Const32 [d])) -> (ConstBool [b2i(uint32(c) > uint32(d))])
 (Greater16U (Const16 [c]) (Const16 [d])) -> (ConstBool [b2i(uint16(c) > uint16(d))])

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

@@ -70,6 +70,34 @@ func rshMask64x32Ext(v int64, s int32) int64 {
 	return v >> uint(s&63)
 }
 
+// --------------- //
+//  signed shifts  //
+// --------------- //
+
+// We do want to generate a test + panicshift for these cases.
+func lshSigned(v8 int8, v16 int16, v32 int32, v64 int64, x int) {
+	// amd64:"TESTB"
+	_ = x << v8
+	// amd64:"TESTW"
+	_ = x << v16
+	// amd64:"TESTL"
+	_ = x << v32
+	// amd64:"TESTQ"
+	_ = x << v64
+}
+
+// We want to avoid generating a test + panicshift for these cases.
+func lshSignedMasked(v8 int8, v16 int16, v32 int32, v64 int64, x int) {
+	// amd64:-"TESTB"
+	_ = x << (v8 & 7)
+	// amd64:-"TESTW"
+	_ = x << (v16 & 15)
+	// amd64:-"TESTL"
+	_ = x << (v32 & 31)
+	// amd64:-"TESTQ"
+	_ = x << (v64 & 63)
+}
+
 // ------------------ //
 //   bounded shifts   //
 // ------------------ //