spirv-remap tool does not function properly with cross-endian binaries

[awilfox]

The test suite for glslang includes a little-endian binary, and on Power9 (big endian) the remap tests fail with:

Testing remapper error handling
1c1
< ID out of range: 4160749568
---
> bad magic number
1c1
< ID not found
---
> bad magic number

A naive patch makes it pass:

--- glslang-14.3.0/StandAlone/spirv-remap.cpp.old	2024-06-25 17:42:43.000000000 -0500
+++ glslang-14.3.0/StandAlone/spirv-remap.cpp	2024-11-22 15:27:02.655869572 -0600
@@ -38,6 +38,7 @@
 #include <cstring>
 #include <stdexcept>
 #include <filesystem>
+#include <byteswap.h>
 
 //
 // Include remapper
@@ -48,6 +49,8 @@
 
     typedef unsigned int SpvWord;
 
+    static const SpvWord MagicNumber = 0x07230203;
+
     // Poor man's basename: given a complete path, return file portion.
     // E.g:
     //      Linux:  /foo/bar/test  -> test
@@ -82,6 +85,7 @@
     void read(std::vector<SpvWord>& spv, const std::string& inFilename, int verbosity)
     {
         std::ifstream fp;
+        bool isNativeEndian = true;
 
         if (verbosity > 0)
             logHandler(std::string("  reading: ") + inFilename);
@@ -97,11 +101,16 @@
         spv.reserve(size_t(fp.tellg()) / sizeof(SpvWord));
         fp.seekg(0, fp.beg);
 
+        char begin = fp.peek();
+        char native_begin = reinterpret_cast<const char*>(&MagicNumber)[0];
+        if (begin != native_begin) isNativeEndian = false;
+
         while (!fp.eof()) {
             SpvWord inWord;
             fp.read((char *)&inWord, sizeof(inWord));
 
             if (!fp.eof()) {
+                if (!isNativeEndian) inWord = __bswap_32(inWord);
                 spv.push_back(inWord);
                 if (fp.fail())
                     errHandler(std::string("error reading file: ") + inFilename);

However, I don't think this is the best way forward. It depends on __bswap_32 which is gcc/clang specific and wouldn't work on Windows. The MagicNumber cast is also ugly though it does not violate strict-aliasing rules.