empty_bytes_shl_optimization.diff

diff -u /tmp/empty_bytesold.yul /tmp/empty_bytesnew.yul
--- /tmp/empty_bytesold.yul	2021-03-17 18:53:59.463802461 +0100
+++ /tmp/empty_bytesnew.yul	2021-03-17 18:53:23.319922761 +0100
@@ -87,20 +87,21 @@
             {
                 let data := sload(array)
                 let oldLen := extract_byte_array_length(data)
-                let _1 := add(oldLen, 1)
+                let _1 := 1
+                let _2 := add(oldLen, _1)
                 switch lt(oldLen, 32)
                 case 0 {
-                    sstore(array, add(mul(2, _1), 1))
+                    sstore(array, add(shl(_1, _2), _1))
                 }
                 default {
-                    switch lt(_1, 32)
+                    switch lt(_2, 32)
                     case 0 {
                         mstore(slot, array)
                         sstore(keccak256(slot, 32), and(not(255), data))
-                        sstore(array, add(mul(2, _1), 1))
+                        sstore(array, add(shl(_1, _2), _1))
                     }
                     default {
-                        sstore(array, or(and(data, not(shr(mul(8, _1), not(0)))), mul(2, _1)))
+                        sstore(array, or(and(data, not(shr(shl(3, _2), not(0)))), shl(_1, _2)))
                     }
                 }
                 let slot_1, offset_1 := storage_array_index_access_bytes(array, oldLen)
@@ -109,7 +110,7 @@
             }
             function extract_byte_array_length(data) -> length
             {
-                length := div(data, 2)
+                length := shr(1, data)
                 let outOfPlaceEncoding := and(data, 1)
                 if iszero(outOfPlaceEncoding) { length := and(length, 0x7f) }
                 if eq(outOfPlaceEncoding, lt(length, 32))
@@ -122,7 +123,7 @@
             function fun_a(var_index) -> var
             {
                 let _1, _2 := storage_array_index_access_bytes(var, var_index)
-                var := shl(248, shr(mul(_2, 8), sload(_1)))
+                var := shl(248, shr(shl(3, _2), sload(_1)))
             }
             function fun_f()
             {
@@ -147,7 +148,7 @@
                     let _1, _2 := array_push_zero_bytes_storage_ptr(var_i)
                     let _3 := 0xff
                     let _4 := sload(_1)
-                    let shiftBits := mul(_2, 8)
+                    let shiftBits := shl(3, _2)
                     let mask := shl(shiftBits, _3)
                     sstore(_1, or(and(_4, not(mask)), and(shl(shiftBits, and(var_i_1, _3)), mask)))
                 }
@@ -166,7 +167,7 @@
                 case 0 {
                     mstore(slot, array)
                     let data := keccak256(slot, 0x20)
-                    slot := add(data, div(index, 0x20))
+                    slot := add(data, shr(5, index))
                 }
                 default { slot := array }
             }

Diff finished.  Wed Mar 17 18:54:06 2021