cmd/compile: remove redundant extension after shift

var x uint64
uint8(x >> 56)

We don't need to generate any code for the uint8().

Update #15090

Change-Id: Ie1ca4e32022dccf7f7bc42d531a285521fb67872
Reviewed-on: https://go-review.googlesource.com/28191
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: randall77

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

@@ -602,6 +602,20 @@
 (Trunc32to16 (And32 (Const32 [y]) x)) && y&0xFFFF == 0xFFFF -> (Trunc32to16 x)
 (Trunc16to8  (And16 (Const16 [y]) x)) && y&0xFF == 0xFF -> (Trunc16to8 x)
 
+(ZeroExt8to64  (Trunc64to8  x:(Rsh64Ux64 _ (Const64 [s])))) && s >= 56 -> x
+(ZeroExt16to64 (Trunc64to16 x:(Rsh64Ux64 _ (Const64 [s])))) && s >= 48 -> x
+(ZeroExt32to64 (Trunc64to32 x:(Rsh64Ux64 _ (Const64 [s])))) && s >= 32 -> x
+(ZeroExt8to32  (Trunc32to8  x:(Rsh32Ux64 _ (Const64 [s])))) && s >= 24 -> x
+(ZeroExt16to32 (Trunc32to16 x:(Rsh32Ux64 _ (Const64 [s])))) && s >= 16 -> x
+(ZeroExt8to16  (Trunc16to8  x:(Rsh16Ux64 _ (Const64 [s])))) && s >= 8 -> x
+
+(SignExt8to64  (Trunc64to8  x:(Rsh64x64 _ (Const64 [s])))) && s >= 56 -> x
+(SignExt16to64 (Trunc64to16 x:(Rsh64x64 _ (Const64 [s])))) && s >= 48 -> x
+(SignExt32to64 (Trunc64to32 x:(Rsh64x64 _ (Const64 [s])))) && s >= 32 -> x
+(SignExt8to32  (Trunc32to8  x:(Rsh32x64 _ (Const64 [s])))) && s >= 24 -> x
+(SignExt16to32 (Trunc32to16 x:(Rsh32x64 _ (Const64 [s])))) && s >= 16 -> x
+(SignExt8to16  (Trunc16to8  x:(Rsh16x64 _ (Const64 [s])))) && s >= 8 -> x
+
 (Slicemask (Const32 [x])) && x > 0 -> (Const32 [-1])
 (Slicemask (Const32 [0]))          -> (Const32 [0])
 (Slicemask (Const64 [x])) && x > 0 -> (Const64 [-1])