diff --git a/Arm/Insts/Common.lean b/Arm/Insts/Common.lean
index 71e877da..04948a16 100644
--- a/Arm/Insts/Common.lean
+++ b/Arm/Insts/Common.lean
@@ -611,13 +611,10 @@ example : rev_vector 32 16 8 0xaabbccdd#32 (by decide)
 /-- Divide bv `vector` into elements, each of size `size`. This function gets
 the `e`'th element from the `vector`. -/
 @[state_simp_rules]
-def elem_get (vector : BitVec n) (e : Nat) (size : Nat)
-  (h: size > 0): BitVec size :=
+def elem_get (vector : BitVec n) (e : Nat) (size : Nat): BitVec size :=
   -- assert (e+1)*size <= n
   let lo := e * size
-  let hi := lo + size - 1
-  have h : hi - lo + 1 = size := by simp only [hi, lo]; omega
-  BitVec.cast h $ extractLsb hi lo vector
+  extractLsb' lo size vector
 
 /-- Divide bv `vector` into elements, each of size `size`. This function sets
 the `e`'th element in the `vector`. -/
@@ -646,7 +643,7 @@ deriving DecidableEq, Repr
 export ShiftInfo (esize elements shift unsigned round accumulate)
 
 @[state_simp_rules]
-def RShr (unsigned : Bool) (value : Int) (shift : Nat) (round : Bool) (h : n > 0)
+def RShr (unsigned : Bool) (value : Int) (shift : Nat) (round : Bool)
   : BitVec n :=
   -- assert shift > 0
   let fn := if unsigned then ushiftRight else sshiftRight
@@ -656,8 +653,7 @@ def RShr (unsigned : Bool) (value : Int) (shift : Nat) (round : Bool) (h : n > 0
       BitVec.ofInt (n + 1) rounded
     else
       BitVec.ofInt (n + 1) value
-  have h₀ : n - 1 - 0 + 1 = n := by omega
-  BitVec.cast h₀ $ extractLsb (n-1) 0 (fn rounded_bv shift)
+  extractLsb' 0 n (fn rounded_bv shift)
 
 @[state_simp_rules]
 def Int_with_unsigned (unsigned : Bool) (value : BitVec n) : Int :=
@@ -669,9 +665,9 @@ def shift_right_common_aux
   if h : info.elements ≤ e then
     result
   else
-    let elem := Int_with_unsigned info.unsigned $ elem_get operand e info.esize info.h
-    let shift_elem := RShr info.unsigned elem info.shift info.round info.h
-    let acc_elem := elem_get operand2 e info.esize info.h + shift_elem
+    let elem := Int_with_unsigned info.unsigned $ elem_get operand e info.esize
+    let shift_elem := RShr info.unsigned elem info.shift info.round
+    let acc_elem := elem_get operand2 e info.esize + shift_elem
     let result := elem_set result e info.esize acc_elem info.h
     have _ : info.elements - (e + 1) < info.elements - e := by omega
     shift_right_common_aux (e + 1) info operand operand2 result
@@ -683,6 +679,13 @@ theorem shift_le (x : Nat) (shift :Nat) :
   simp only [Nat.shiftRight_eq_div_pow]
   exact Nat.div_le_self x (2 ^ shift)
 
+-- FIXME: should this be upstreamed?
+theorem extractLsb'_ofNat (x n : Nat) (lo size : Nat) :
+  extractLsb' lo size (BitVec.ofNat n x) = .ofNat size ((x % 2^n) >>> lo) := by
+  apply eq_of_getLsbD_eq
+  intro ⟨i, _lt⟩
+  simp [BitVec.ofNat]
+
 @[state_simp_rules]
 theorem shift_right_common_aux_64_2_tff (operand : BitVec 128)
   (shift : Nat) (result : BitVec 128):
@@ -691,8 +694,8 @@ theorem shift_right_common_aux_64_2_tff (operand : BitVec 128)
      unsigned := true, round := false, accumulate := false,
      h := (by omega)}
     operand 0#128 result =
-  (ushiftRight (extractLsb 127 64 operand) shift)
-    ++ (ushiftRight (extractLsb 63 0 operand) shift) := by
+  (ushiftRight (extractLsb' 64 64 operand) shift)
+    ++ (ushiftRight (extractLsb' 0 64 operand) shift) := by
   unfold shift_right_common_aux
   simp only [minimal_theory, bitvec_rules]
   unfold shift_right_common_aux
@@ -728,16 +731,17 @@ theorem shift_right_common_aux_64_2_tff (operand : BitVec 128)
              -- Eliminating casting functions
              Int.ofNat_eq_coe, ofInt_natCast, ofNat_toNat
     ]
-  generalize (extractLsb 127 64 operand) = x; simp at x
-  generalize (extractLsb 63 0 operand) = y; simp at y
-  have h0 : ∀ (z : BitVec 64), extractLsb 63 0 ((zeroExtend 65 z).ushiftRight shift)
+  simp only [reduceExtracLsb', BitVec.zero_add]
+  generalize (extractLsb' 64 64 operand) = x
+  generalize (extractLsb' 0 64 operand) = y
+  have h0 : ∀ (z : BitVec 64), extractLsb' 0 64 ((zeroExtend 65 z).ushiftRight shift)
     = z.ushiftRight shift := by
     intro z
     simp only [ushiftRight, toNat_truncate]
     have h1: z.toNat % 2 ^ 65 = z.toNat := by omega
     simp only [h1]
     simp only [Std.Tactic.BVDecide.Normalize.BitVec.ofNatLt_reduce]
-    simp only [Nat.sub_zero, Nat.reduceAdd, BitVec.extractLsb_ofNat, Nat.shiftRight_zero]
+    simp only [Nat.sub_zero, Nat.reduceAdd, extractLsb'_ofNat, Nat.shiftRight_zero]
     have h2 : z.toNat >>> shift % 2 ^ 65 = z.toNat >>> shift := by
       refine Nat.mod_eq_of_lt ?h3
       have h4 : z.toNat >>> shift ≤ z.toNat := by exact shift_le z.toNat shift
@@ -780,10 +784,10 @@ theorem shift_right_common_aux_32_4_fff (operand : BitVec 128)
       unsigned := false, round := false, accumulate := false,
       h := (by omega) }
       operand 0#128 result =
-  (sshiftRight (extractLsb 127 96 operand) shift)
-    ++ (sshiftRight (extractLsb 95 64 operand) shift)
-    ++ (sshiftRight (extractLsb 63 32 operand) shift)
-    ++ (sshiftRight (extractLsb 31 0 operand) shift) := by
+  (sshiftRight (extractLsb' 96 32 operand) shift)
+    ++ (sshiftRight (extractLsb' 64 32 operand) shift)
+    ++ (sshiftRight (extractLsb' 32 32 operand) shift)
+    ++ (sshiftRight (extractLsb' 0 32 operand) shift) := by
   unfold shift_right_common_aux
   simp only [minimal_theory, bitvec_rules]
   unfold shift_right_common_aux
@@ -823,20 +827,19 @@ theorem shift_right_common_aux_32_4_fff (operand : BitVec 128)
              -- Eliminating casting functions
              ofInt_eq_signExtend
     ]
-  generalize extractLsb 31 0 operand = a; simp at a
-  generalize extractLsb 63 32 operand = b; simp at b
-  generalize extractLsb 95 64 operand = c; simp at c
-  generalize extractLsb 127 96 operand = d; simp at d
+  generalize extractLsb' 0 32 operand = a
+  generalize extractLsb' 32 32 operand = b
+  generalize extractLsb' 64 32 operand = c
+  generalize extractLsb' 96 32 operand = d
   have h : ∀ (x : BitVec 32),
-    extractLsb 31 0 ((signExtend 33 x).sshiftRight shift)
+    extractLsb' 0 32 ((signExtend 33 x).sshiftRight shift)
     = x.sshiftRight shift := by
     intros x
     apply eq_of_getLsbD_eq; intros i; simp at i
     simp only [getLsbD_sshiftRight]
-    simp only [Nat.sub_zero, Nat.reduceAdd, getLsbD_extract, Nat.zero_add,
-               getLsbD_sshiftRight, getLsbD_signExtend]
-    simp only [show (i : Nat) ≤ 31 by omega,
-               decide_True, Bool.true_and]
+    simp only [getLsbD_extractLsb', Fin.is_lt, decide_True,
+               Nat.zero_add, getLsbD_sshiftRight,
+               getLsbD_signExtend, Bool.true_and]
     simp only [show ¬33 ≤ (i : Nat) by omega,
                decide_False, Bool.not_false, Bool.true_and]
     simp only [show ¬32 ≤ (i : Nat) by omega,
@@ -876,7 +879,7 @@ def shift_left_common_aux
   if h : info.elements ≤ e then
     result
   else
-    let elem := elem_get operand e info.esize info.h
+    let elem := elem_get operand e info.esize
     let shift_elem := elem <<< info.shift
     let result := elem_set result e info.esize shift_elem info.h
     have _ : info.elements - (e + 1) < info.elements - e := by omega
@@ -891,8 +894,8 @@ theorem shift_left_common_aux_64_2 (operand : BitVec 128)
      unsigned := unsigned, round := round, accumulate := accumulate,
      h := (by omega)}
     operand result =
-  (extractLsb 127 64 operand <<< shift)
-    ++ (extractLsb 63 0 operand <<< shift) := by
+  (extractLsb' 64 64 operand <<< shift)
+    ++ (extractLsb' 0 64 operand <<< shift) := by
   unfold shift_left_common_aux
   simp only [minimal_theory, bitvec_rules]
   unfold shift_left_common_aux
diff --git a/Arm/Insts/DPSFP/Advanced_simd_copy.lean b/Arm/Insts/DPSFP/Advanced_simd_copy.lean
index aaa094f1..a2146a9e 100644
--- a/Arm/Insts/DPSFP/Advanced_simd_copy.lean
+++ b/Arm/Insts/DPSFP/Advanced_simd_copy.lean
@@ -44,7 +44,7 @@ def exec_dup_element (inst : Advanced_simd_copy_cls) (s : ArmState) : ArmState :
     let elements := datasize / esize
     let operand := read_sfp idxdsize inst.Rn s
     have h₀ : esize > 0 := by apply zero_lt_shift_left_pos (by decide)
-    let element := elem_get operand index esize h₀
+    let element := elem_get operand index esize
     let result := dup_aux 0 elements esize element (BitVec.zero datasize) h₀
     -- State Updates
     let s := write_pc ((read_pc s) + 4#64) s
@@ -81,7 +81,7 @@ def exec_ins_element (inst : Advanced_simd_copy_cls) (s : ArmState) : ArmState :
     let operand := read_sfp idxdsize inst.Rn s
     let result := read_sfp 128 inst.Rd s
     have h₀ : esize > 0 := by apply zero_lt_shift_left_pos (by decide)
-    let elem := elem_get operand src_index esize h₀
+    let elem := elem_get operand src_index esize
     let result := elem_set result dst_index esize elem h₀
     -- State Updates
     let s := write_pc ((read_pc s) + 4#64) s
@@ -123,7 +123,7 @@ def exec_smov_umov (inst : Advanced_simd_copy_cls) (s : ArmState) (signed : Bool
     -- if index == 0 then CheckFPEnabled64 else CheckFPAdvSIMDEnabled64
     let operand := read_sfp idxdsize inst.Rn s
     have h₀ : esize > 0 := by apply zero_lt_shift_left_pos (by decide)
-    let element := elem_get operand index esize h₀
+    let element := elem_get operand index esize
     let result := if signed then signExtend datasize element else zeroExtend datasize element
     -- State Updates
     let s := write_pc ((read_pc s) + 4#64) s
diff --git a/Arm/Insts/DPSFP/Advanced_simd_permute.lean b/Arm/Insts/DPSFP/Advanced_simd_permute.lean
index 5ccc6bff..95a2920c 100644
--- a/Arm/Insts/DPSFP/Advanced_simd_permute.lean
+++ b/Arm/Insts/DPSFP/Advanced_simd_permute.lean
@@ -22,8 +22,8 @@ def trn_aux (p : Nat) (pairs : Nat) (esize : Nat) (part : Nat)
     result
   else
     let idx_from := 2 * p + part
-    let op1_part := elem_get operand1 idx_from esize h
-    let op2_part := elem_get operand2 idx_from esize h
+    let op1_part := elem_get operand1 idx_from esize
+    let op2_part := elem_get operand2 idx_from esize
     let result := elem_set result (2 * p) esize op1_part h
     let result := elem_set result (2 * p + 1) esize op2_part h
     have h₁ : pairs - (p + 1) < pairs - p := by omega
diff --git a/Arm/Insts/DPSFP/Advanced_simd_scalar_copy.lean b/Arm/Insts/DPSFP/Advanced_simd_scalar_copy.lean
index 3f2b25c2..effdd228 100644
--- a/Arm/Insts/DPSFP/Advanced_simd_scalar_copy.lean
+++ b/Arm/Insts/DPSFP/Advanced_simd_scalar_copy.lean
@@ -27,8 +27,7 @@ def exec_advanced_simd_scalar_copy
     let idxdsize := 64 <<< (lsb inst.imm5 4).toNat
     let esize := 8 <<< size
     let operand := read_sfp idxdsize inst.Rn s
-    have h : esize > 0 := by apply zero_lt_shift_left_pos (by decide)
-    let result := elem_get operand index.toNat esize h
+    let result := elem_get operand index.toNat esize
     -- State Updates
     let s := write_pc ((read_pc s) + 4#64) s
     let s := write_sfp esize inst.Rd result s
diff --git a/Arm/Insts/DPSFP/Advanced_simd_table_lookup.lean b/Arm/Insts/DPSFP/Advanced_simd_table_lookup.lean
index b032904c..16024f23 100644
--- a/Arm/Insts/DPSFP/Advanced_simd_table_lookup.lean
+++ b/Arm/Insts/DPSFP/Advanced_simd_table_lookup.lean
@@ -35,10 +35,10 @@ def tblx_aux (i : Nat) (elements : Nat) (indices : BitVec datasize)
     result
   else
     have h₁ : 8 > 0 := by decide
-    let index := (elem_get indices i 8 h₁).toNat
+    let index := (elem_get indices i 8).toNat
     let result :=
       if index < 16 * regs then
-        let val := elem_get table index 8 h₁
+        let val := elem_get table index 8
         elem_set result i 8 val h₁
       else
         result
diff --git a/Arm/Insts/DPSFP/Advanced_simd_three_different.lean b/Arm/Insts/DPSFP/Advanced_simd_three_different.lean
index 53800ea8..5a64fe3f 100644
--- a/Arm/Insts/DPSFP/Advanced_simd_three_different.lean
+++ b/Arm/Insts/DPSFP/Advanced_simd_three_different.lean
@@ -37,8 +37,8 @@ def pmull_op (e : Nat) (esize : Nat) (elements : Nat) (x : BitVec n)
   if h₀ : elements <= e then
     result
   else
-    let element1 := elem_get x e esize H
-    let element2 := elem_get y e esize H
+    let element1 := elem_get x e esize
+    let element2 := elem_get y e esize
     let elem_result := polynomial_mult element1 element2
     have h₁ : esize + esize = 2 * esize := by omega
     have h₂ : 2 * esize > 0 := by omega
diff --git a/Arm/Insts/DPSFP/Advanced_simd_three_same.lean b/Arm/Insts/DPSFP/Advanced_simd_three_same.lean
index a532ac04..7da3cc42 100644
--- a/Arm/Insts/DPSFP/Advanced_simd_three_same.lean
+++ b/Arm/Insts/DPSFP/Advanced_simd_three_same.lean
@@ -25,8 +25,8 @@ def binary_vector_op_aux (e : Nat) (elems : Nat) (esize : Nat)
     result
   else
     have h₁ : e < elems := by omega
-    let element1 := elem_get x e esize H
-    let element2 := elem_get y e esize H
+    let element1 := elem_get x e esize
+    let element2 := elem_get y e esize
     let elem_result := op element1 element2
     let result := elem_set result e esize elem_result H
     have ht1 : elems - (e + 1) < elems - e := by omega
diff --git a/Proofs/AES-GCM/GCMInitV8Sym.lean b/Proofs/AES-GCM/GCMInitV8Sym.lean
index b4d287b3..dcfe8edc 100644
--- a/Proofs/AES-GCM/GCMInitV8Sym.lean
+++ b/Proofs/AES-GCM/GCMInitV8Sym.lean
@@ -94,5 +94,6 @@ theorem gcm_init_v8_program_correct (s0 sf : ArmState)
     , shift_right_common_aux_32_4_fff
     , DPSFP.AdvSIMDExpandImm
     , DPSFP.dup_aux_0_4_32]
+    simp only [BitVec.extractLsb'_eq_extractLsb.symm]
     generalize read_mem_bytes 16 (r (StateField.GPR 1#5) s0) s0 = Hinit
     sorry