handle rounding artifacts in alphas_cumprod

Author: rmatif

stable-diffusion.cpp

@@ -716,8 +716,24 @@ class StableDiffusionGGML {
 
         auto comp_vis_denoiser = std::dynamic_pointer_cast<CompVisDenoiser>(denoiser);
         if (comp_vis_denoiser) {
+            float* alphas_ptr = (float*)alphas_cumprod_tensor->data;
+            bool needs_regen  = false;
+
+            for (int i = 0; i < TIMESTEPS; ++i) {
+                float alpha = alphas_ptr[i];
+                if (!std::isfinite(alpha) || alpha <= 0.0f || alpha >= 1.0f) {
+                    needs_regen = true;
+                    break;
+                }
+            }
+
+            if (needs_regen) {
+                calculate_alphas_cumprod(alphas_ptr);
+            }
+            
             for (int i = 0; i < TIMESTEPS; i++) {
-                comp_vis_denoiser->sigmas[i]     = std::sqrt((1 - ((float*)alphas_cumprod_tensor->data)[i]) / ((float*)alphas_cumprod_tensor->data)[i]);
+                float alpha = std::min(std::max(alphas_ptr[i], 1e-12f), 1.0f - 1e-12f);
+                comp_vis_denoiser->sigmas[i]     = std::sqrt((1.0f - alpha) / alpha);
                 comp_vis_denoiser->log_sigmas[i] = std::log(comp_vis_denoiser->sigmas[i]);
             }
         }