cmd/compile: fold CMPconst and SHR

Fold the comparison when the SHR result is small.

Useful for:
- murmur mix like hashing where higher bits are desirable, i.e. hash = uint32(i * C) >> 18
- integer log2 via DeBruijn sequence: http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn

Change-Id: If70ae18cb86f4cc83ab6213f88ced03cc4986156
Reviewed-on: https://go-review.googlesource.com/21514
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1101,6 +1101,8 @@
 (CMPQconst (MOVBQZX _) [c]) && 0xFF < c -> (FlagLT_ULT)
 (CMPQconst (MOVWQZX _) [c]) && 0xFFFF < c -> (FlagLT_ULT)
 (CMPQconst (MOVLQZX _) [c]) && 0xFFFFFFFF < c -> (FlagLT_ULT)
+(CMPLconst (SHRLconst _ [c]) [n]) && 0 <= n && 0 < c && c <= 32 && (1<<uint64(32-c)) <= uint64(n) -> (FlagLT_ULT)
+(CMPQconst (SHRQconst _ [c]) [n]) && 0 <= n && 0 < c && c <= 64 && (1<<uint64(64-c)) <= uint64(n) -> (FlagLT_ULT)
 (CMPQconst (ANDQconst _ [m]) [n]) && 0 <= m && m < n -> (FlagLT_ULT)
 (CMPLconst (ANDLconst _ [m]) [n]) && 0 <= int32(m) && int32(m) < int32(n) -> (FlagLT_ULT)
 (CMPWconst (ANDWconst _ [m]) [n]) && 0 <= int16(m) && int16(m) < int16(n) -> (FlagLT_ULT)

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -2869,6 +2869,22 @@ func rewriteValueAMD64_OpAMD64CMPLconst(v *Value, config *Config) bool {
 		v.reset(OpAMD64FlagGT_UGT)
 		return true
 	}
+	// match: (CMPLconst (SHRLconst _ [c]) [n])
+	// cond: 0 <= n && 0 < c && c <= 32 && (1<<uint64(32-c)) <= uint64(n)
+	// result: (FlagLT_ULT)
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64SHRLconst {
+			break
+		}
+		c := v_0.AuxInt
+		n := v.AuxInt
+		if !(0 <= n && 0 < c && c <= 32 && (1<<uint64(32-c)) <= uint64(n)) {
+			break
+		}
+		v.reset(OpAMD64FlagLT_ULT)
+		return true
+	}
 	// match: (CMPLconst (ANDLconst _ [m]) [n])
 	// cond: 0 <= int32(m) && int32(m) < int32(n)
 	// result: (FlagLT_ULT)
@@ -3107,6 +3123,22 @@ func rewriteValueAMD64_OpAMD64CMPQconst(v *Value, config *Config) bool {
 		v.reset(OpAMD64FlagLT_ULT)
 		return true
 	}
+	// match: (CMPQconst (SHRQconst _ [c]) [n])
+	// cond: 0 <= n && 0 < c && c <= 64 && (1<<uint64(64-c)) <= uint64(n)
+	// result: (FlagLT_ULT)
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64SHRQconst {
+			break
+		}
+		c := v_0.AuxInt
+		n := v.AuxInt
+		if !(0 <= n && 0 < c && c <= 64 && (1<<uint64(64-c)) <= uint64(n)) {
+			break
+		}
+		v.reset(OpAMD64FlagLT_ULT)
+		return true
+	}
 	// match: (CMPQconst (ANDQconst _ [m]) [n])
 	// cond: 0 <= m && m < n
 	// result: (FlagLT_ULT)

test/checkbce.go

@@ -52,6 +52,18 @@ func f5(a []int) {
 	}
 }
 
+func f6(a [32]int, b [64]int, i int) {
+	useInt(a[uint32(i*0x07C4ACDD)>>27])
+	useInt(b[uint64(i*0x07C4ACDD)>>58])
+	useInt(a[uint(i*0x07C4ACDD)>>59])
+
+	// The following bounds should removed as they can overflow.
+	useInt(a[uint32(i*0x106297f105d0cc86)>>26]) // ERROR "Found IsInBounds$"
+	useInt(b[uint64(i*0x106297f105d0cc86)>>57]) // ERROR "Found IsInBounds$"
+	useInt(a[int32(i*0x106297f105d0cc86)>>26])  // ERROR "Found IsInBounds$"
+	useInt(b[int64(i*0x106297f105d0cc86)>>57])  // ERROR "Found IsInBounds$"
+}
+
 func g1(a []int) {
 	for i := range a {
 		a[i] = i