Origin: https://github.com/silnrsi/grcompiler/commit/19c55e009386d62468d97948db581cff08962bbb
From: Tim Eves <tim_eves@sil.org>
Date: Sun, 16 Aug 2020 13:05:41 +0700
Subject: Fixes #31: ResolveToFeatureID assumed little endian

The code attempted to read a series of up to 4 bytes into a 32 bit value
and left align anything less than 4 bytes.
However it used union type aliasing and assumed little endian host memory
layout. On a bigendian system this reverses the byte order and results
in corrupted Graphtie Feature IDs being written to the TTF.
---
diff --git a/compiler/GdlExpression.cpp b/compiler/GdlExpression.cpp
index 08fe847..e973c52 100644
--- a/compiler/GdlExpression.cpp
+++ b/compiler/GdlExpression.cpp
@@ -488,15 +488,13 @@ bool GdlStringExpression::ResolveToFeatureID(unsigned int * pnRet)
 	if (m_staValue.length() > 4)
 		return false;
 
-	union {
-		char rgch[4];
-		unsigned int n;
-	} featid;
 	// The way we do the assignments ensures the characters are left-aligned
 	// in the 4-byte integer (ie, occupying the most significant bytes).
-	for (size_t ich = 0; ich < 4; ich++)
-		featid.rgch[3-ich] = (ich < m_staValue.length()) ? m_staValue[ich] : 0;
-	*pnRet = featid.n;
+	unsigned int id = 0;
+	for(auto c : m_staValue) { id <<= 8; id += c; }
+	id <<= 8*(4-m_staValue.length());
+
+	*pnRet = id;
 	return true;
 }