From 44ec97993a28ba109f98c4514e27cef813720399 Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
Date: Mon, 13 Oct 2025 17:53:42 -0700
Subject: [PATCH] compiler: add and use unreachable() macro

C23 defines unreachable() as a macro in <stddef.h>. For earlier
versions of gcc, __builtin_unreachable() is possible.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 asm/error.c        | 18 +++++++++++-------
 asm/nasm.c         | 29 ++++++++++++++++-------------
 configure.ac       |  1 +
 include/compiler.h |  8 ++++++++
 4 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/asm/error.c b/asm/error.c
index a9671bc8c..0e17ffef1 100644
--- a/asm/error.c
+++ b/asm/error.c
@@ -13,17 +13,21 @@ unsigned int debug_nasm;        /* Debugging messages? */
 unsigned int opt_verbose_info;  /* Informational messages? */
 
 /* Common function body */
-#define nasm_do_error(_sev,_flags)				\
+#define nasm_do_error(_sev,_flags)                              \
     do {                                                        \
+        const errflags nde_severity = (_sev);                   \
+        const errflags nde_flags = nde_severity | (_flags);     \
         va_list ap;                                             \
         va_start(ap, fmt);                                      \
-        if ((_sev) >= ERR_CRITICAL)                             \
-            nasm_verror_critical((_sev)|(_flags), fmt, ap);     \
-        else							\
-            nasm_verror((_sev)|(_flags), fmt, ap);              \
+        if (nde_severity >= ERR_CRITICAL) {                     \
+            nasm_verror_critical(nde_flags, fmt, ap);           \
+            unreachable();                                      \
+        } else {						\
+            nasm_verror(nde_flags, fmt, ap);                    \
+            if (nde_severity >= ERR_FATAL)                      \
+                unreachable();                                  \
+        }                                                       \
         va_end(ap);                                             \
-        if ((_sev) >= ERR_FATAL)                                \
-            abort();                                            \
     } while (0)
 
 /*
diff --git a/asm/nasm.c b/asm/nasm.c
index 49fa00eb7..2a209154c 100644
--- a/asm/nasm.c
+++ b/asm/nasm.c
@@ -1874,23 +1874,25 @@ static const char no_file_name[] = "nasm"; /* What to print if no file name */
  */
 static_fatal_func die_hard(errflags true_type, errflags severity)
 {
-    fflush(NULL);
+    if (true_type < ERR_PANIC || !abort_on_panic) {
+        fflush(NULL);
 
-    if (true_type == ERR_PANIC && abort_on_panic)
-        abort();
+        if (ofile) {
+            fclose(ofile);
+            if (!keep_all)
+                remove(outname);
+            ofile = NULL;
+        }
 
-    if (ofile) {
-        fclose(ofile);
-        if (!keep_all)
-            remove(outname);
-        ofile = NULL;
+        if (severity & ERR_USAGE)
+            usage();
+
+        /* Terminate immediately */
+        exit(true_type - ERR_FATAL + 1);
     }
 
-    if (severity & ERR_USAGE)
-        usage();
-
-    /* Terminate immediately */
-    exit(true_type - ERR_FATAL + 1);
+    while (1)
+        abort();
 }
 
 /*
@@ -1950,6 +1952,7 @@ fatal_func nasm_verror_critical(errflags severity, const char *fmt, va_list args
     fputc('\n', error_file);
 
     die_hard(true_type, severity);
+    unreachable();
 }
 
 /**
diff --git a/configure.ac b/configure.ac
index 448eeb291..acd7461bf 100644
--- a/configure.ac
+++ b/configure.ac
@@ -224,6 +224,7 @@ AC_CHECK_FUNCS(sysconf)
 AC_CHECK_FUNCS([access _access faccessat])
 
 PA_HAVE_FUNC(__builtin_expect,(1,1))
+PA_HAVE_FUNC(__builtin_unreachable,())
 
 PA_FUNC_SNPRINTF
 PA_FUNC_VSNPRINTF
diff --git a/include/compiler.h b/include/compiler.h
index d4cb71617..21025d7fa 100644
--- a/include/compiler.h
+++ b/include/compiler.h
@@ -469,4 +469,12 @@ static inline unsigned int watcom_switch_hack(uint64_t x)
 # define default case BOGUS_CASE: default
 #endif
 
+#ifndef unreachable              /* C23 defines as a macro in <stddef.h> */
+# ifdef HAVE___BUILTIN_UNREACHABLE
+#  define unreachable() __builtin_unreachable()
+# else
+#  define unreachable() do { abort(); } while(1)
+# endif
+#endif
+
 #endif	/* NASM_COMPILER_H */