From bcbdec74f4c8c3104a35b0bf87deec7c0fa5f14c Mon Sep 17 00:00:00 2001
From: Jongsoo Park <jongsoo@fb.com>
Date: Thu, 26 Dec 2019 14:18:33 -0800
Subject: [PATCH] add more comments on handling small scale handling; remove
 redundant if (#232)

Summary:
Pull Request resolved: https://github.com/pytorch/FBGEMM/pull/232

As title

Reviewed By: dskhudia

Differential Revision: D19231440

fbshipit-source-id: fe4db92bc6b5dd2822dbdac75f29b779d80cee65
---
 src/PackAWithQuantRowOffset.cc | 2 +-
 src/QuantUtils.cc              | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/PackAWithQuantRowOffset.cc b/src/PackAWithQuantRowOffset.cc
index fb90101332..f99ba27989 100644
--- a/src/PackAWithQuantRowOffset.cc
+++ b/src/PackAWithQuantRowOffset.cc
@@ -46,7 +46,7 @@ PackAWithQuantRowOffset<T, accT>::PackAWithQuantRowOffset(
   if (!cpuinfo_initialize()) {
     throw std::runtime_error("Failed to initialize cpuinfo!");
   }
-  if (scale_ == 0.0f || std::isinf(1.0f / scale_)) {
+  if (scale_ == 0.0f) {
     throw std::runtime_error("scale cannot be zero");
   }
   if (std::isinf(1.0f / scale_)) {
diff --git a/src/QuantUtils.cc b/src/QuantUtils.cc
index 2a2d379b5b..be911e3608 100644
--- a/src/QuantUtils.cc
+++ b/src/QuantUtils.cc
@@ -42,7 +42,9 @@ TensorQuantizationParams ChooseQuantizationParams(
   // final number to reflect the actual number used during quantization.
   float scale = (static_cast<double>(max) - min) / (qmax - qmin);
   // If scale is 0 or too small so its reciprocal is infinity, we arbitrary
-  // adjust the scale to 0.1
+  // adjust the scale to 0.1 . We want to avoid scale's reciprocal being infinity
+  // because some of fbgemm code pre-computes scale's reciprocal to do
+  // multiplication instead of division in the time critical part of code.
   if (scale == 0.0f || isinf(1.0f / scale)) {
     scale = 0.1;
   }