From: Matthew Fernandez <matthew.fernandez@gmail.com>
Date: Sun, 20 Jun 2021 21:09:41 +0000 (-0700)
Subject: add a macro for optimizing unreachable code
X-Git-Tag: 2.48.0~31^2~3
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=0a1324f28dc0250c75cd4ea82b5bd56d04ef68d3;p=graphviz

add a macro for optimizing unreachable code
---

diff --git a/lib/cgraph/CMakeLists.txt b/lib/cgraph/CMakeLists.txt
index f52e93d9a..b1466d2d7 100644
--- a/lib/cgraph/CMakeLists.txt
+++ b/lib/cgraph/CMakeLists.txt
@@ -12,6 +12,7 @@ add_library(cgraph SHARED
     itos.h
     sprint.h
     strcasecmp.h
+    unreachable.h
 
     # Source files
     agerror.c
diff --git a/lib/cgraph/Makefile.am b/lib/cgraph/Makefile.am
index 9d5204525..04317540b 100644
--- a/lib/cgraph/Makefile.am
+++ b/lib/cgraph/Makefile.am
@@ -8,7 +8,7 @@ pkgconfigdir = $(libdir)/pkgconfig
 AM_CPPFLAGS = -I$(top_srcdir)/lib -I$(top_srcdir)/lib/cdt
 
 pkginclude_HEADERS = cgraph.h
-noinst_HEADERS = agxbuf.h cghdr.h itos.h sprint.h strcasecmp.h
+noinst_HEADERS = agxbuf.h cghdr.h itos.h sprint.h strcasecmp.h unreachable.h
 noinst_LTLIBRARIES = libcgraph_C.la
 lib_LTLIBRARIES = libcgraph.la
 pkgconfig_DATA = libcgraph.pc
diff --git a/lib/cgraph/cgraph.vcxproj b/lib/cgraph/cgraph.vcxproj
index 7fa787f35..03f0d3d3a 100644
--- a/lib/cgraph/cgraph.vcxproj
+++ b/lib/cgraph/cgraph.vcxproj
@@ -104,6 +104,7 @@ win_flex -oscan.c scan.l</Command>
     <ClInclude Include="itos.h" />
     <ClInclude Include="sprint.h" />
     <ClInclude Include="strcasecmp.h" />
+    <ClInclude Include="unreachable.h" />
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="agerror.c" />
diff --git a/lib/cgraph/cgraph.vcxproj.filters b/lib/cgraph/cgraph.vcxproj.filters
index f6d120770..7d5911412 100644
--- a/lib/cgraph/cgraph.vcxproj.filters
+++ b/lib/cgraph/cgraph.vcxproj.filters
@@ -33,6 +33,9 @@
     <ClInclude Include="strcasecmp.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="unreachable.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="agerror.c">
diff --git a/lib/cgraph/unreachable.h b/lib/cgraph/unreachable.h
new file mode 100644
index 000000000..f3d69b730
--- /dev/null
+++ b/lib/cgraph/unreachable.h
@@ -0,0 +1,47 @@
+#pragma once
+
+#include <assert.h>
+#include <stdlib.h>
+
+/** Marker for a point in code which execution can never reach.
+ *
+ * As a C11 function, this could be thought of as:
+ *
+ *   _Noreturn void UNREACHABLE(void);
+ *
+ * Calling this teaches the compiler that the call site can never be executed,
+ * and it can optimize with this assumption in mind. This can be used to explain
+ * that a switch is exhaustive:
+ *
+ *   switch (…) {
+ *   default: UNREACHABLE();
+ *   …remaining cases that cover all possibilities…
+ *   }
+ *
+ * or that a function coda can be omitted:
+ *
+ *   int foo(void) {
+ *     while (always_true()) {
+ *     }
+ *     UNREACHABLE();
+ *   }
+ */
+#ifdef __GNUC__
+#define UNREACHABLE()                                                          \
+  do {                                                                         \
+    assert(0 && "unreachable");                                                \
+    __builtin_unreachable();                                                   \
+  } while (0)
+#elif defined(_MSC_VER)
+#define UNREACHABLE()                                                          \
+  do {                                                                         \
+    assert(0 && "unreachable");                                                \
+    __assume(0);                                                               \
+  } while (0)
+#else
+#define UNREACHABLE()                                                          \
+  do {                                                                         \
+    assert(0 && "unreachable");                                                \
+    abort();                                                                   \
+  } while (0)
+#endif