From 65af215162f87099f949b31c5c401152e70a0914 Mon Sep 17 00:00:00 2001
From: Matthew Fernandez <matthew.fernandez@gmail.com>
Date: Wed, 9 Mar 2022 18:57:17 -0800
Subject: [PATCH] avoid use of '__builtin_unreachable' in 'UNREACHABLE' macro
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There was some discussion on !2503¹ around the effect of `__builtin_unreachable`
and whether we are really confident applying such a strong compiler hint to
complex control flow logic that is, in some cases, only partially understood.
This change softens the effect of the `UNREACHABLE` macro to a reliable program
abort rather than undefined behavior if mistakenly tagged unreachable code is
reached. The result is slightly less efficient but safer code.

¹ https://gitlab.com/graphviz/graphviz/-/merge_requests/2503#note_866261546
---
 lib/cgraph/unreachable.h | 22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

diff --git a/lib/cgraph/unreachable.h b/lib/cgraph/unreachable.h
index f3d69b730..919d71534 100644
--- a/lib/cgraph/unreachable.h
+++ b/lib/cgraph/unreachable.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <assert.h>
+#include <stdio.h>
 #include <stdlib.h>
 
 /** Marker for a point in code which execution can never reach.
@@ -9,9 +10,7 @@
  *
  *   _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:
+ * This can be used to explain that a switch is exhaustive:
  *
  *   switch (…) {
  *   default: UNREACHABLE();
@@ -26,22 +25,9 @@
  *     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");                                                \
+    fprintf(stderr, "%s:%d: claimed unreachable code was reached", __FILE__,   \
+            __LINE__);                                                         \
     abort();                                                                   \
   } while (0)
-#endif
-- 
2.40.0