Merge pull request #4598 from bjorng/bjorn/compiler/opportunistic-war…

…nings/OTP-17260

Support suppressing opportunistic compiler warnings

lib/compiler/doc/src/compile.xml

@@ -772,29 +772,43 @@ module.beam: module.erl \
           </item>
         </taglist>
 
-	<p>Another class of warnings is generated by the compiler
-	  during optimization and code generation. They warn about
-	  patterns that will never match (such as <c>a=b</c>), guards
-	  that always evaluate to false, and expressions that
-	  always fail (such as <c>atom+42</c>).</p>
-	  <p>Those warnings cannot be disabled (except by
-	  disabling all warnings).</p>
+	<p>Other kinds of warnings are <em>opportunistic
+	warnings</em>. They are generated when the compiler happens to
+	notice potential issues during optimization and code
+	generation.</p>
+
+	<note>
+	  <p>The compiler does not warn for expressions that it
+          does not attempt to optimize. For example, the compiler will
+          emit a warning for <c>1/0</c> but not for <c>X/0</c>,
+          because <c>1/0</c> is a constant expression that the compiler
+          will attempt to evaluate.</p>
+
+          <p>The absence of warnings does not mean that there are no
+          remaining errors in the code.</p>
+        </note>
+
+        <p>Opportunistic warnings can be disabled using the following
+        options:</p>
+
+        <taglist>
+          <tag><c>nowarn_opportunistic</c></tag>
+          <item><p>Disable all opportunistic warnings.</p></item>
+
+          <tag><c>nowarn_failed</c></tag>
+          <item><p>Disable warnings for expressions that will always
+          fail (such as <c>atom+42</c>).</p></item>
+
+          <tag><c>nowarn_ignored</c></tag>
+          <item><p>Disable warnings for expressions whose
+          values are ignored.</p></item>
+
+          <tag><c>nowarn_nomatch</c></tag>
+          <item><p>Disable warnings for patterns that will never match
+          (such as <c>a=b</c>) and for guards that always evaluate to
+          <c>false</c>.</p></item>
+        </taglist>
 
-	  <note>
-	    <p>The compiler does not warn for expressions that it
-          does not attempt to optimize. For example, the compiler tries
-          to evaluate <c>1/0</c>, detects that it will cause an
-	  exception, and emits a warning. However,
-	  the compiler is silent about the similar expression,
-	  <c>X/0</c>, because of the variable in it. Thus, the compiler does
-	  not even try to evaluate and therefore it emits no warnings.</p>
-          </note>
-
-	<warning>
-          <p>The absence of warnings does not mean that
-	    there are no remaining errors in the code.</p>
-        </warning>
-
 	<note>
 	<p>All options, except the include path
 	  (<c>{i,Dir}</c>), can also be given in the file with attribute

lib/compiler/src/compile.erl

@@ -514,6 +514,7 @@ run_eprof({_,Fun}, Code, _, St) ->
     Fun(Code, St).
 
 comp_ret_ok(Code, #compile{warnings=Warn0,module=Mod,options=Opts}=St) ->
+    Warn1 = filter_warnings(Warn0, Opts),
     case werror(St) of
         true ->
             case member(report_warnings, Opts) of
@@ -525,7 +526,7 @@ comp_ret_ok(Code, #compile{warnings=Warn0,module=Mod,options=Opts}=St) ->
             end,
             comp_ret_err(St);
         false ->
-            Warn = messages_per_file(Warn0),
+            Warn = messages_per_file(Warn1),
             report_warnings(St#compile{warnings = Warn}),
             Ret1 = case member(binary, Opts) andalso
 		       not member(no_code_generation, Opts) of
@@ -1720,6 +1721,41 @@ report_warnings(#compile{options=Opts,warnings=Ws0}) ->
 	false -> ok
     end.
 
+%%%
+%%% Filter warnings.
+%%%
+
+filter_warnings(Ws, Opts) ->
+    Ignore = ignore_tags(Opts, sets:new([{version,2}])),
+    filter_warnings_1(Ws, Ignore).
+
+filter_warnings_1([{Source,Ws0}|T], Ignore) ->
+    Ws = [W || W <- Ws0, not ignore_warning(W, Ignore)],
+    [{Source,Ws}|filter_warnings_1(T, Ignore)];
+filter_warnings_1([], _Ignore) -> [].
+
+ignore_warning({_Location,Pass,{Category,_}}, Ignore) ->
+    IgnoreMod = case Pass of
+                    v3_core -> true;
+                    sys_core_fold -> true;
+                    v3_kernel -> true;
+                    _ -> false
+                end,
+    IgnoreMod andalso sets:is_element(Category, Ignore);
+ignore_warning(_, _) -> false.
+
+ignore_tags([nowarn_opportunistic|_], _Ignore) ->
+    sets:from_list([failed,ignored,nomatch], [{version,2}]);
+ignore_tags([nowarn_failed|Opts], Ignore) ->
+    ignore_tags(Opts, sets:add_element(failed, Ignore));
+ignore_tags([nowarn_ignored|Opts], Ignore) ->
+    ignore_tags(Opts, sets:add_element(ignored, Ignore));
+ignore_tags([nowarn_nomatch|Opts], Ignore) ->
+    ignore_tags(Opts, sets:add_element(nomatch, Ignore));
+ignore_tags([_|Opts], Ignore) ->
+    ignore_tags(Opts, Ignore);
+ignore_tags([], Ignore) -> Ignore.
+
 %% erlfile(Dir, Base) -> ErlFile
 %% outfile(Base, Extension, Options) -> OutputFile
 %% objfile(Base, Target, Options) -> ObjFile