Revert "compile: prefer an AND instead of SHR+SHL instructions"

This reverts commit 9ec7074.

Reason for revert: broke s390x (copysign, abs) and arm64 (bitfield) tests.

Change-Id: I16c1b389c062e8c4aa5de079f1d46c9b25b0db52
Reviewed-on: https://go-review.googlesource.com/c/go/+/193850
Run-TryBot: Martin Möhrmann <[email protected]>
Reviewed-by: Agniva De Sarker <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: martisch

src/cmd/compile/internal/ssa/gen/ARM64.rules

@@ -1863,8 +1863,9 @@
 (XORshiftLL <t> [c] (UBFX [bfc] x) x2) && c < 32 && t.Size() == 4 && bfc == armBFAuxInt(32-c, c)
 	-> (EXTRWconst [32-c] x2 x)
 
-// Rewrite special pairs of shifts to AND.
-// On ARM64 the bitmask can fit into an instruction.
+// Generic rules rewrite certain AND to a pair of shifts.
+// However, on ARM64 the bitmask can fit into an instruction.
+// Rewrite it back to AND.
 (SRLconst [c] (SLLconst [c] x)) && 0 < c && c < 64 -> (ANDconst [1<<uint(64-c)-1] x) // mask out high bits
 (SLLconst [c] (SRLconst [c] x)) && 0 < c && c < 64 -> (ANDconst [^(1<<uint(c)-1)] x) // mask out low bits
 

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

@@ -542,6 +542,14 @@
 (Slicemask (Const64 [x])) && x > 0 -> (Const64 [-1])
 (Slicemask (Const64 [0]))          -> (Const64 [0])
 
+// Rewrite AND of consts as shifts if possible, slightly faster for 64 bit operands
+// leading zeros can be shifted left, then right
+(And64 <t> (Const64 [y]) x) && nlz(y) + nto(y) == 64 && nto(y) >= 32
+  -> (Rsh64Ux64 (Lsh64x64 <t> x (Const64 <t> [nlz(y)])) (Const64 <t> [nlz(y)]))
+// trailing zeros can be shifted right, then left
+(And64 <t> (Const64 [y]) x) && nlo(y) + ntz(y) == 64 && ntz(y) >= 32
+  -> (Lsh64x64 (Rsh64Ux64 <t> x (Const64 <t> [ntz(y)])) (Const64 <t> [ntz(y)]))
+
 // simplifications often used for lengths.  e.g. len(s[i:i+5])==5
 (Sub(64|32|16|8) (Add(64|32|16|8) x y) x) -> y
 (Sub(64|32|16|8) (Add(64|32|16|8) x y) y) -> x

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

@@ -81,7 +81,7 @@ func abs32(x float32) float32 {
 
 // Check that it's using integer registers
 func copysign(a, b, c float64) {
-	// amd64:"BTRQ\t[$]63","ANDQ","ORQ"
+	// amd64:"BTRQ\t[$]63","SHRQ\t[$]63","SHLQ\t[$]63","ORQ"
 	// s390x:"CPSDR",-"MOVD"         (no integer load/store)
 	// ppc64:"FCPSGN"
 	// ppc64le:"FCPSGN"
@@ -100,7 +100,7 @@ func copysign(a, b, c float64) {
 	// s390x:"LNDFR\t",-"MOVD\t"     (no integer load/store)
 	sink64[2] = math.Float64frombits(math.Float64bits(a) | 1<<63)
 
-	// amd64:"ANDQ","ORQ"
+	// amd64:-"SHLQ\t[$]1",-"SHRQ\t[$]1","SHRQ\t[$]63","SHLQ\t[$]63","ORQ"
 	// s390x:"CPSDR\t",-"MOVD\t"     (no integer load/store)
 	// ppc64:"FCPSGN"
 	// ppc64le:"FCPSGN"