README.md: Document memory debugger pitfalls
(more specifically, the need to disable libjpeg-turbo's SIMD extensions
when testing with valgrind, MSan, etc.)
Addresses a concern expressed in #365
diff --git a/README.md b/README.md
index 2d4db9f..c61b855 100644
--- a/README.md
+++ b/README.md
@@ -342,3 +342,15 @@
function in those cases. This causes performance to drop by as much as 40%.
It is therefore strongly advised that you use the slow integer forward DCT
whenever encoding images with a JPEG quality of 98 or higher.
+
+
+Memory Debugger Pitfalls
+========================
+
+Valgrind and Memory Sanitizer (MSan) can generate false positives
+(specifically, incorrect reports of uninitialized memory accesses) when used
+with libjpeg-turbo's SIMD extensions. It is generally recommended that the
+SIMD extensions be disabled, either by passing an argument of `-DWITH_SIMD=0`
+to `cmake` when configuring the build or by setting the environment variable
+`JSIMD_FORCENONE` to `1` at run time, when testing libjpeg-turbo with Valgrind,
+MSan, or other memory debuggers.
