Disable -avx512f on LLVM < 5.0.0 to avoid bug 30542

Prior to this commit, we could accidentally trigger llvm bug 30542
(https://bugs.llvm.org/show_bug.cgi?id=30542) on a cpu with the
`avx512f` feature. See WallarooLabs/wally#1925
for one particular example of this occurring.

This commit automagically disables `avx512f` if the LLVM in use is
below version 5.0.0 to work around the issue.

README.md

@@ -277,6 +277,8 @@ There is experimental support for building with LLVM 4.0.1 or 5.0.0,
 but this may result in decreased performance or crashes in generated
 applications.
 
+NOTE: If LLVM version < 5.0.0 is used, cpu feature `avx512f` is diabled automagically to avoid [LLVM bug 30542](https://bugs.llvm.org/show_bug.cgi?id=30542) otherwise the compiler crashes during the optimization phase.
+
 Compiling Pony is only possible on x86 and ARM (either 32 or 64 bits).
 
 ## Building on Linux

src/libponyc/codegen/codegen.c

@@ -978,12 +978,31 @@ bool codegen_pass_init(pass_opt_t* opt)
 {
   if(opt->features != NULL)
   {
+#if PONY_LLVM < 500
+    // Disable -avx512f on LLVM < 5.0.0 to avoid bug https://bugs.llvm.org/show_bug.cgi?id=30542
+    size_t temp_len = strlen(opt->features) + 9;
+    char* temp_str = (char*)ponyint_pool_alloc_size(temp_len);
+    snprintf(temp_str, temp_len, "%s,-avx512f", opt->features);
+
+    opt->features = temp_str;
+#endif
+
     opt->features = LLVMCreateMessage(opt->features);
+
+
+#if PONY_LLVM < 500
+    // free memory for temp_str
+    ponyint_pool_free_size(temp_len, temp_str);
+#endif
   } else {
     if((opt->cpu == NULL) && (opt->triple == NULL))
       opt->features = LLVMGetHostCPUFeatures();
     else
+#if PONY_LLVM < 500
+      opt->features = LLVMCreateMessage("-avx512f");
+#else
       opt->features = LLVMCreateMessage("");
+#endif
   }
 
   // Default triple, cpu and features.

src/libponyc/codegen/host.cc

@@ -46,6 +46,11 @@ char* LLVMGetHostCPUFeatures()
   for(auto it = features.begin(); it != features.end(); it++)
     buf_size += (*it).getKey().str().length() + 2; // plus +/- char and ,/null
 
+#if PONY_LLVM < 500
+  // Add extra buffer space for llvm bug workaround
+  buf_size += 9;
+#endif
+
   char* buf = (char*)malloc(buf_size);
   pony_assert(buf != NULL);
   buf[0] = 0;
@@ -64,6 +69,11 @@ char* LLVMGetHostCPUFeatures()
       strcat(buf, ",");
   }
 
+#if PONY_LLVM < 500
+  // Disable -avx512f on LLVM < 5.0.0 to avoid bug https://bugs.llvm.org/show_bug.cgi?id=30542
+  strcat(buf, ",-avx512f");
+#endif
+
   return buf;
 }