Fix sampling errors due to float rounding errors on large-vocab models

Author: turboderp

exllamav2/exllamav2_ext/cpp/sampling.cpp

@@ -833,7 +833,12 @@ int multinomial_cpu
     while (true)
     {
         if (accum >= random) break;
-        if (idx == num_candidates - 1) break;
+        if (idx == num_candidates - 1)
+        {
+            // Roll back in case the sampled probability is exactly zero
+            while (idx > 0 && temp_probs[idx] == 0.0f) idx--;
+            break;
+        }
         idx++;
         accum += temp_probs[idx];
     }

exllamav2/exllamav2_ext/cpp/util.h

@@ -14,6 +14,7 @@
 #define DBGF(__x) printf("%s: %f\n", #__x, __x)
 #define DBGF2(__x, __y) printf("%s, %s: %f, %f\n", #__x, #__y, __x, __y)
 #define DBGF3(__x, __y, __z) printf("%s, %s, %s: %f, %f, %f\n", #__x, #__y, #__z, __x, __y, __z)
+#define DBGF4(__x, __y, __z, __w) printf("%s, %s, %s, %s: %f, %f, %f, %f\n", #__x, #__y, #__z, #__w, __x, __y, __z, __w)
 #define DBGIF(__x, __y) printf("%s, %s: %i, %f\n", #__x, #__y, __x, __y)
 
 #define TIME_START \

exllamav2/exllamav2_ext/ext_sampling.cpp

@@ -230,7 +230,24 @@ std::vector<float> sample_basic
             random_s = powf(random, expf(-skew));
         }
 
-        multinomial_cpu(num_candidates, temp_probs, temp_indices, random_s);
+//        {
+//            float sum = 0.0f;
+//            float pmin = temp_probs[0];
+//            float pmax = pmin;
+//            for (int i = 0; i < num_candidates; ++i)
+//            {
+//                if (temp_probs[i] < pmin) pmin = temp_probs[i];
+//                if (temp_probs[i] > pmax) pmax = temp_probs[i];
+//                sum += temp_probs[i];
+//            }
+//            DBGF4(pmin, pmax, sum, random_s);
+//        }
+
+        // Scale random sampling point a little to account for FP32 rounding errors during softmax. Probs
+        // can potentially sum to slightly less than 1 for large-vocab models
+        float random_s_adj = random_s * 0.9998;
+
+        multinomial_cpu(num_candidates, temp_probs, temp_indices, random_s_adj);
 
         output_tokens[i][0] = temp_indices[0];
         output_probs[i][0] = temp_probs[0];