Avoid overflow in WebRtcSpl_Sqrt

webrtc/common_audio/signal_processing/spl_sqrt.c

Index: webrtc/common_audio/signal_processing/spl_sqrt.c
diff --git a/webrtc/common_audio/signal_processing/spl_sqrt.c b/webrtc/common_audio/signal_processing/spl_sqrt.c
index 24db4f822ca153ceebd85d436135970a651ac559..3e01dcbd7ddade860dd4d252e625e2bf0dc0a4c0 100644
--- a/webrtc/common_audio/signal_processing/spl_sqrt.c
+++ b/webrtc/common_audio/signal_processing/spl_sqrt.c
@@ -139,6 +139,16 @@ int32_t WebRtcSpl_Sqrt(int32_t value)
 
     A = value;
 
+    // The convention in this function is to calculate sqrt(abs(A)). Negate the
+    // input if it is negative.
+    if (A == WEBRTC_SPL_WORD32_MIN) {
+        // This number cannot be held in an int32_t after negating.
+        // Map it to the maximum positive value.
+        A = WEBRTC_SPL_WORD32_MAX;
+    } else if (A < 0) {
+        A = -A;
+    }

[kwiberg-webrtc, 2016/02/11 09:28:10]

The optimizer may already be doing this for you, but it might not be a bad idea
to hide the most uncommon condition (WORD32_MIN) behind the less uncommon one
(negative). Also, you can avoid loss of precision by the decrement if you simply
return the correct result:

if (A < 0) {
  if (A == WEBRTC_SPL_WORD32_MIN) {
    COMPILE_ASSERT(WEBRTC_SPL_WORD32_MIN == -2147483648);
    return 46341;  // sqrt(2147483648) = 46340.950012...
  }
  A = -A;
} else if (A == 0) {
  return 0;  // sqrt(0) = 0
}

[hlundin-webrtc, 2016/02/11 22:02:29]

Much better. Done.

+
     if (A == 0)
         return (int32_t)0; // sqrt(0) = 0