[WebAssebmly] Remove reloc ordering constraint

[sbc100]

Summary

The MC layer doesn't currently emit relocations in offset
order for the entire code section so this check was causing
failures on the wasm waterfall.

Perhaps we can re-instate this check if we devide the relocations
per-function, or add extra ordering the MC object writer.

[sbc100]

• debugging

[ncw]

Oops, thanks very much!

It was passing for all the checked-in tests, so I guess there's a test missing then. Do you have an example of what input file creates out-of-order relocs?

[sbc100]

On the waterfall it was happening in libc++abi.a. The example I found was cxa_vector.cpp.o. I did think about trying to generate a new test for this case, but decided not to prioritize it. I'll attache that object file. If you fancy coming up with a minimal reproducer that would be welcome new test. I imagine we might be able to re-enforce this once we switch to per-function and per-segment relocs.

[sbc100]

cxa_vector.cpp.o6 KBDownload

[ncw]

Thanks, I'll have a quick look out of curiosity and see if I can work out how the relocs are appearing out of order (it may affect your work to group the relocs, since you'll need to "chunk" them into per-segment batches which is similar to getting them ordered).

llvm/trunk/lib/Object/WasmObjectFile.cpp

if (SectionCode == wasm::WASM_SEC_CUSTOM) {
     Section = findCustomSectionByName(Name);
   } else {
     Section = findSectionByType(SectionCode);
   }
   if (!Section)
     return make_error<GenericBinaryError>("Invalid section code",
                                           object_error::parse_failed);
   uint32_t RelocCount = readVaruint32(Ptr);
-  uint32_t LastOffset = 0;
   uint32_t EndOffset = Section->Content.size();
   while (RelocCount--) {
     wasm::WasmRelocation Reloc = {};
     Reloc.Type = readVaruint32(Ptr);
     Reloc.Offset = readVaruint32(Ptr);
     Reloc.Index = readVaruint32(Ptr);
     switch (Reloc.Type) {
     case wasm::R_WEBASSEMBLY_FUNCTION_INDEX_LEB:
while (RelocCount--) {
 
     // Relocations must fit inside the section, and must appear in order.  They
     // also shouldn't overlap a function/element boundary, but we don't bother
     // to check that.
     uint64_t Size = 5;
     if (Reloc.Type == wasm::R_WEBASSEMBLY_TABLE_INDEX_I32 ||
         Reloc.Type == wasm::R_WEBASSEMBLY_MEMORY_ADDR_I32)
       Size = 4;
-    if (Reloc.Offset < LastOffset || Reloc.Offset + Size > EndOffset)
+    if (Reloc.Offset + Size > EndOffset)
       return make_error<GenericBinaryError>("Bad relocation offset",
                                             object_error::parse_failed);
-    LastOffset = Reloc.Offset;
 
     Section->Relocations.push_back(Reloc);
   }
   if (Ptr != End)
     return make_error<GenericBinaryError>("Reloc section ended prematurely",
                                           object_error::parse_failed);
   return Error::success();
 }