[PATCH] D95052: [WebAssembly][lld] R_WASM_TABLE_NUMBER_LEB is variable-width

[Andy Wingo via Phabricator via llvm-commits]

wingo created this revision.
Herald added subscribers: ecnelises, sunfish, jgravelle-google, sbc100, dschuff.
wingo requested review of this revision.
Herald added subscribers: llvm-commits, aheejin.
Herald added a project: LLVM.

WebAssembly 1.0 only supports a table-number immediate operand of 0x00.
As we add support for multiple tables with a corresponding relocation,
continue to allow generation of one-byte table number references, so
that the "normal" case of just one table with the number 0 can be
written as 0x00.  This allows the compiler to output relocations for
call_indirect while targetting WebAssembly 1.0, simplifying the
configuration space.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D95052

Files:
  lld/wasm/InputChunks.cpp


Index: lld/wasm/InputChunks.cpp
===================================================================
--- lld/wasm/InputChunks.cpp
+++ lld/wasm/InputChunks.cpp
@@ -105,8 +105,13 @@
       llvm_unreachable("unknown relocation type");
     }
 
-    if (bytesRead && bytesRead != paddedLEBWidth)
-      warn("expected LEB at relocation site be 5/10-byte padded");
+    if (rel.Type == R_WASM_TABLE_NUMBER_LEB) {
+      if (bytesRead < 1 || bytesRead > 5)
+        warn("expected LEB at relocation site be 1-5 bytes long");
+    } else {
+      if (bytesRead && bytesRead != paddedLEBWidth)
+        warn("expected LEB at relocation site be 5/10-byte padded");
+    }
 
     if (rel.Type != R_WASM_GLOBAL_INDEX_LEB &&
         rel.Type != R_WASM_GLOBAL_INDEX_I32) {
@@ -153,9 +158,14 @@
     case R_WASM_GLOBAL_INDEX_LEB:
     case R_WASM_EVENT_INDEX_LEB:
     case R_WASM_MEMORY_ADDR_LEB:
-    case R_WASM_TABLE_NUMBER_LEB:
       encodeULEB128(value, loc, 5);
       break;
+    case R_WASM_TABLE_NUMBER_LEB: {
+      uint32_t bytesRead = 0;
+      decodeULEB128(loc, &bytesRead);
+      encodeULEB128(value, loc, bytesRead);
+      break;
+    }
     case R_WASM_MEMORY_ADDR_LEB64:
       encodeULEB128(value, loc, 10);
       break;
@@ -254,7 +264,6 @@
   case R_WASM_GLOBAL_INDEX_LEB:
   case R_WASM_EVENT_INDEX_LEB:
   case R_WASM_MEMORY_ADDR_LEB:
-  case R_WASM_TABLE_NUMBER_LEB:
   case R_WASM_TABLE_INDEX_SLEB:
   case R_WASM_MEMORY_ADDR_SLEB:
     return 5;
@@ -272,6 +281,21 @@
   return writeCompressedReloc(buf, rel, value);
 }
 
+static bool relocHasPaddedWidth(const WasmRelocation &rel) {
+  return rel.Type != R_WASM_TABLE_NUMBER_LEB;
+}
+
+static uint32_t getRelocInputWidth(const uint8_t *buf,
+                                   const WasmRelocation &rel) {
+  if (relocHasPaddedWidth(rel)) {
+    return getRelocWidthPadded(rel);
+  } else {
+    uint32_t relocSize = 0;
+    decodeULEB128(buf + rel.Offset, &relocSize);
+    return relocSize;
+  }
+}
+
 // Relocations of type LEB and SLEB in the code section are padded to 5 bytes
 // so that a fast linker can blindly overwrite them without needing to worry
 // about the number of bytes needed to encode the values.
@@ -303,7 +327,7 @@
     LLVM_DEBUG(dbgs() << "  region: " << (rel.Offset - lastRelocEnd) << "\n");
     compressedFuncSize += rel.Offset - lastRelocEnd;
     compressedFuncSize += getRelocWidth(rel, file->calcNewValue(rel, tombstone));
-    lastRelocEnd = rel.Offset + getRelocWidthPadded(rel);
+    lastRelocEnd = rel.Offset + getRelocInputWidth(secStart, rel);
   }
   LLVM_DEBUG(dbgs() << "  final region: " << (end - lastRelocEnd) << "\n");
   compressedFuncSize += end - lastRelocEnd;
@@ -344,7 +368,7 @@
     memcpy(buf, lastRelocEnd, chunkSize);
     buf += chunkSize;
     buf += writeCompressedReloc(buf, rel, file->calcNewValue(rel, tombstone));
-    lastRelocEnd = secStart + rel.Offset + getRelocWidthPadded(rel);
+    lastRelocEnd = secStart + rel.Offset + getRelocInputWidth(secStart, rel);
   }
 
   unsigned chunkSize = end - lastRelocEnd;


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D95052.317889.patch
Type: text/x-patch
Size: 3042 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20210120/c8368cfb/attachment.bin>