diff --git a/src/losses/ctc-gpu.jl b/src/losses/ctc-gpu.jl
index 65d8b9a4fb..f3d6592628 100644
--- a/src/losses/ctc-gpu.jl
+++ b/src/losses/ctc-gpu.jl
@@ -7,7 +7,8 @@
 using Flux
 using Statistics
 using CUDA
-using DelimitedFiles
+
+const MAX_THREADS = 256
 
 function log_plus_f(p1, p2)
   
@@ -51,7 +52,7 @@ function computeAlphaKernel(probs, labelSize, uttLength, repeats, labelsWithoutB
   # Fill in first column (time step)
   i = tid
   while i <= last - start
-    alpha[start + i] = probs[labels[start + i]]
+    alpha[start+i, 1] = probs[labels[start+i], 1]
     i += blockDim().x
   end
   
@@ -59,16 +60,13 @@ function computeAlphaKernel(probs, labelSize, uttLength, repeats, labelsWithoutB
   
   # Fill in coefficients for each time step
   for t=2:T
-    startCurCol = (t-1) * S
-    startPrevCol = (t-2) * S
-    startProbCol = (t-1) * div(length(probs), T)
     
     # Corner-case checking
     if tid == 1 && !(1 < S - 2*(T-t) - 1)
       if start == 0
-        alpha[startCurCol + 1] = probs[startProbCol + blankLabel] + alpha[startPrevCol + 1]
+        alpha[1, t] = probs[blankLabel, t] + alpha[1, t-1]
       elseif start == 1
-        alpha[startCurCol + 1] = alpha[startPrevCol + 1]
+        alpha[1, t] = alpha[1, t-1]
       end
     end
     
@@ -79,16 +77,16 @@ function computeAlphaKernel(probs, labelSize, uttLength, repeats, labelsWithoutB
     idx = tid+1
     while idx <= S
       
-      prevSum = log_plus_f(alpha[startPrevCol + idx], alpha[startPrevCol + idx-1])
+      prevSum = log_plus_f(alpha[idx, t-1], alpha[idx-1, t-1])
       
       if labels[idx] != blankLabel && idx != 2 && labels[idx] != labels[idx-2]
-        prevSum = log_plus_f(prevSum, alpha[startPrevCol + idx-2])
+        prevSum = log_plus_f(prevSum, alpha[idx-2, t-1])
       end
       
       if idx < S - 2*(T-t) - 1
-        alpha[idx + startCurCol] = -Inf32
+        alpha[idx, t] = -Inf32
       else
-        alpha[startCurCol + idx] = prevSum + probs[startProbCol + labels[idx]]
+        alpha[idx, t] = prevSum + probs[labels[idx], t]
       end
     
       idx += blockDim().x
@@ -122,31 +120,23 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
   
   sync_threads()
   
-  
-  startCurCol = (T-1)*S
-  startProbCol = (T-1) * div(length(probs), T)
-  
   i = tid
   
   # Calculate coefficients for last column (time step)
   # then determine alpha and beta product
   while i <= last - start + 1
-
-    beta[startCurCol + i + start] = 0
-    output[startCurCol + i + start] = beta[startCurCol + i + start] + alphas[startCurCol + i + start]
+    beta[i+start, T] = 0
+    output[i+start, T] = beta[i+start, T] + alphas[i+start, T]
     i += blockDim().x
   end
   
   sync_threads()
   
   # Fill in `accum` for last column (time step)
-  if tid == 1
-    startAccCol = startProbCol
-    startOutputCol = startCurCol
-    
+  if tid == 1    
     for i=1:S
       labelIdx = labels[i]
-      accum[startAccCol + labelIdx] = log_plus_f(accum[startAccCol + labelIdx], output[startOutputCol + i])
+      accum[labelIdx, T] = log_plus_f(accum[labelIdx, T], output[i, T])
     end
   end
   
@@ -154,20 +144,16 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
   
   # Fill in `grad` for last column (time step)
   idx = tid
-  # while idx <= CUDA.div_fast(Float32(length(grad)), Float32(T))
   while idx <= size(grad, 1)
-#     
-    startProbCol = (T - 1) * div(length(probs), T)
-    startOutputCol = (T - 1) * S
     
     s = -Inf32
+    
     for i=1:S
-      s = log_plus_f(s, output[startOutputCol + i])
+      s = log_plus_f(s, output[i, T])
     end
     
     # ∂L/∂a (where a is activation before logsoftmax)
-    # grad[startProbCol + idx] = CUDA.exp(probs[startProbCol + idx]) - CUDA.exp(accum[startProbCol + idx] - s)
-    grad[idx, T] = CUDA.exp(probs[startProbCol + idx]) - CUDA.exp(accum[startProbCol + idx] - s)
+    grad[idx, T] = CUDA.exp(probs[idx, T]) - CUDA.exp(accum[idx, T] - s)
     idx += blockDim().x
   end
   
@@ -176,28 +162,23 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
   # Fill in the rest of the coefficients
   t = T-1
   while t >= 1
-  
-    startCurCol = (t-1)*S
-    startNextCol = t*S
-    startProbCol = t * div(length(probs), T)
-
     if t < T
       
       idx = tid
       while idx <= S-1
         
-        nextSum = log_plus_f(beta[startNextCol + idx] + probs[startProbCol + labels[idx]],
-          beta[startNextCol + idx+1] + probs[startProbCol + labels[idx+1]])
+        nextSum = log_plus_f(beta[idx, t+1] + probs[labels[idx], t+1],
+          beta[idx+1, t+1] + probs[labels[idx+1], t+1])
         
         if labels[idx] != blankLabel && idx != S-1 && labels[idx] != labels[idx+2]
           nextSum = log_plus_f(nextSum,
-            beta[startNextCol + idx + 2] + probs[startProbCol + labels[idx+2]])
+            beta[idx + 2, t+1] + probs[labels[idx+2], t+1])
         end
         
         if idx > 2*t
-          beta[idx + startCurCol] = -Inf32
+          beta[idx, t] = -Inf32
         else
-          beta[idx + startCurCol] = nextSum
+          beta[idx, t] = nextSum
             
         end
         
@@ -207,14 +188,14 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
       sync_threads()
       
       if tid == 1 && last == S
-        beta[startCurCol + S] = beta[startNextCol + S] + probs[startProbCol + blankLabel]
+        beta[S, t] = beta[S, t] + probs[blankLabel, t+1]
       end
       
       sync_threads()
       
       idx = tid
       while idx <= S
-        output[startCurCol + idx] = alphas[idx+startCurCol] + beta[startCurCol + idx]
+        output[idx, t] = alphas[idx, t] + beta[idx, t]
         idx += blockDim().x
       end
       
@@ -226,14 +207,10 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
     
     # Calculate accumulated alpha-beta products for each label class for
     # each time step; used in calculating gradients
-    if tid == 1
-    
-      startAccCol = (t-1) * div(length(accum), T)
-      startOutputCol = (t-1) * S
-      
+    if tid == 1      
       for i=1:S
         labelIdx = labels[i]
-        accum[startAccCol + labelIdx] = log_plus_f(accum[startAccCol + labelIdx], output[startOutputCol + i])
+        accum[labelIdx, t] = log_plus_f(accum[labelIdx, t], output[i, t])
       end
     end
     
@@ -243,17 +220,15 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
     
     # Calculate gradients
     while idx <= size(grad, 1)
-#     
-      startProbCol = (t - 1) * div(length(probs), T)
-      startOutputCol = (t - 1) * S
       
       s = -Inf32
+      
       for i=1:S
-        s = log_plus_f(s, output[startOutputCol + i])
+        s = log_plus_f(s, output[i, t])
       end
       
       # ∂L/∂a (where a is activation before logsoftmax)
-      grad[idx, t] = CUDA.exp(probs[startProbCol + idx]) - CUDA.exp(accum[startProbCol + idx] - s)
+      grad[idx, t] = CUDA.exp(probs[idx, t]) - CUDA.exp(accum[idx, t] - s)
       idx += blockDim().x
     end
     
@@ -266,20 +241,15 @@ function computeBetasAndGradKernel(probs, labelSize, uttLength,
   return nothing
 end
 
+# methods for `ctc_` helper function
 ctc(ŷ::CuArray, y::Array) = ctc_(ŷ, y)[1] |> mean
-
 ctc(ŷ::Array, y::CuArray) = ctc_(CuArray(ŷ), collect(y))[1] |> mean
-
 ctc(ŷ::CuArray, y::CuArray) = ctc_(ŷ, collect(y))[1] |> mean
-
-# methods for `ctc_` helper function
-ctc_(ŷ::Array, y::CuArray) =  ctc_(CuArray(ŷ), y)
+ctc_(ŷ::Array, y::CuArray) =  ctc_(CuArray(ŷ), collect(y))
 
 function ctc_(ŷ::CuArray, y)
   
   ŷ = logsoftmax(ŷ)
-  if any(isinf.(ŷ)) error("Inf in yhat") end
-  if any(isnan.(ŷ)) error("NaN in yhat") end
   
   blank = size(ŷ, 1)
   labels = [Base.argmax(y[:,i]) for i in 1:size(y, 2)]
@@ -289,23 +259,25 @@ function ctc_(ŷ::CuArray, y)
     push!(z′, label)
     push!(z′, blank)
   end
+  
   T = size(ŷ, 2)
   U′ = 2*length(z) + 1
-  alphas = CUDA.fill(log(zero(ŷ[1])), T * U′)
-  betas = copy(alphas)
-  output = copy(alphas)
+  
+  alphas = CUDA.fill(log(zero(ŷ[1])), U′, T)
+  betas = CUDA.fill(log(zero(ŷ[1])), U′, T)
+  output = CUDA.fill(log(zero(ŷ[1])), U′, T)
   
   nRepeats = countRepeats(labels)
+  nThreads = min(U′, MAX_THREADS)
 
-  # 1 block with `U′` threads
-  @cuda blocks=1 threads=U′ computeAlphaKernel(ŷ, length(z), size(ŷ,2), nRepeats, CuArray(z), CuArray(z′), alphas, blank)
+  @cuda blocks=1 threads=nThreads computeAlphaKernel(ŷ, length(z), size(ŷ,2), nRepeats, CuArray(z), CuArray(z′), alphas, blank)
   
   grads = CUDA.fill(log(zero(ŷ[1])), size(ŷ))
-  accum = CUDA.fill(log(zero(ŷ[1])), length(ŷ))
+  accum = CUDA.fill(log(zero(ŷ[1])), size(ŷ))
   
-  @cuda blocks=1 threads=U′ computeBetasAndGradKernel(ŷ, length(z), size(ŷ,2), nRepeats, CuArray(z′), alphas, betas, output, accum, grads, blank)
+  @cuda blocks=1 threads=nThreads computeBetasAndGradKernel(ŷ, length(z), size(ŷ,2), nRepeats, CuArray(z′), alphas, betas, output, accum, grads, blank)
   
-  ls = reshape(collect(output), U′, T)
+  ls = collect(output)
   ls = vec(-1 .* [logsum(ls[:,i]) for i in 1:size(ls, 2)])
   
   ŷ = alphas = betas = output = accum = nothing