Suppress warnings about constant expressions on MSVC (GH-4317)

Adds utility code sections to toggle C compiler warnings for the duration of the utility code.

2 files changed, 31 insertions, 0 deletions

diff --git a/Cython/Compiler/Code.py b/Cython/Compiler/Code.py
index f02bbae27..a2affbe89 100644
--- a/Cython/Compiler/Code.py
+++ b/Cython/Compiler/Code.py
@@ -1143,7 +1143,9 @@ class GlobalState(object):
         'cleanup_globals',
         'cleanup_module',
         'main_method',
+        'utility_code_pragmas',  # silence some irrelevant warnings in utility code
         'utility_code_def',
+        'utility_code_pragmas_end',  # clean-up the utility_code_pragmas
         'end'
     ]
 
@@ -1241,6 +1243,18 @@ class GlobalState(object):
         code.put(util.format_code(util.impl))
         code.putln("")
 
+        #
+        # utility code pragmas
+        #
+        code = self.parts['utility_code_pragmas']
+        util = UtilityCode.load_cached("UtilityCodePragmas", "ModuleSetupCode.c")
+        code.putln(util.format_code(util.impl))
+        code.putln("")
+        code = self.parts['utility_code_pragmas_end']
+        util = UtilityCode.load_cached("UtilityCodePragmasEnd", "ModuleSetupCode.c")
+        code.putln(util.format_code(util.impl))
+        code.putln("")
+
     def __getitem__(self, key):
         return self.parts[key]
 
diff --git a/Cython/Utility/ModuleSetupCode.c b/Cython/Utility/ModuleSetupCode.c
index 6c2141893..dcd081742 100644
--- a/Cython/Utility/ModuleSetupCode.c
+++ b/Cython/Utility/ModuleSetupCode.c
@@ -1796,3 +1796,20 @@ static void __Pyx_FastGilFuncInit(void) {
 }
 
 #endif
+
+///////////////////// UtilityCodePragmas /////////////////////////
+
+#if _MSC_VER
+#pragma warning( push )
+/* Warning 4127: conditional expression is constant
+ * Cython uses constant conditional expressions to allow in inline functions to be optimized at
+ * compile-time, so this warning is not useful
+ */
+#pragma warning( disable : 4127 )
+#endif
+
+///////////////////// UtilityCodePragmasEnd //////////////////////
+
+#if _MSV_VER
+#pragma warning( pop )  /* undo whatever Cython has done to warnings */
+#endif