diff --git a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
--- a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
@@ -510,8 +510,14 @@
   const RangeInt LongMax = BVF.getMaxValue(LongTy).getLimitedValue();
   const RangeInt LongLongMax = BVF.getMaxValue(LongLongTy).getLimitedValue();
 
-  const RangeInt UCharMax =
-      BVF.getMaxValue(ACtx.UnsignedCharTy).getLimitedValue();
+  // Set UCharMax to min of int or uchar maximum value.
+  // The C standard states that the arguments of functions like isalpha must
+  // be representable as an unsigned char. Their type is 'int', so the max
+  // value of the argument should be min(UCharMax, IntMax). This just happen
+  // to be true for commonly used and well tested instruction set
+  // architectures, but not for others.
+  const RangeInt UCharMax = 
+      std::min(BVF.getMaxValue(ACtx.UnsignedCharTy).getLimitedValue(), IntMax);
 
   // The platform dependent value of EOF.
   // Try our best to parse this from the Preprocessor, otherwise fallback to -1.