[llvm] [WebAssembly] Don't assert on a non-zero call_indirect table index (PR #210120)

[via llvm-commits]

llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-webassembly

Author: Jonas Devlieghere (JDevlieghere)

<details>
<summary>Changes</summary>

WebAssemblyInstPrinter::printInst asserted that a non-symbol table operand of call_indirect is the immediate 0, on the assumption that only MVP single-table compilation units reach the printer. That is true for code generated by the compiler, but the printer is shared by the disassembler.

The disassembler must produce output for any byte sequence it is pointed at and shouldn't abort on the operand values it decodes. A non-zero immediate table index is both a valid encoding, from a multi-table module, and a routine result of best-effort disassembly of a range that is not all code, which is how I hit this in LLDB.

Print the table operand when it is a symbol or a non-zero immediate, and omit it only for the implicit table 0. This makes the output lossless and unambiguous, matching how a table symbol is already printed. MVP and table-symbol output are unchanged, so there is no effect on the compiler's assembly.

---
Full diff: https://github.com/llvm/llvm-project/pull/210120.diff


2 Files Affected:

- (modified) llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp (+9-8) 
- (modified) llvm/test/MC/Disassembler/WebAssembly/wasm.txt (+5) 


``````````diff
diff --git a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp
index 8cee0788def0a..5bfaffba3e578 100644
--- a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp
@@ -60,19 +60,20 @@ void WebAssemblyInstPrinter::printInst(const MCInst *MI, uint64_t Address,
   case WebAssembly::RET_CALL_INDIRECT:
   case WebAssembly::CALL_INDIRECT_S:
   case WebAssembly::RET_CALL_INDIRECT_S: {
-    // A special case for call_indirect (and ret_call_indirect), if the table
-    // operand is a symbol: the order of the type and table operands is inverted
-    // in the text format relative to the binary format.  Otherwise if table the
-    // operand isn't a symbol, then we have an MVP compilation unit, and the
-    // table shouldn't appear in the output.
+    // A special case for call_indirect (and ret_call_indirect): the order of
+    // the type and table operands is inverted in the text format relative to
+    // the binary format. The table operand is omitted when it refers to the
+    // default table 0 of an MVP compilation unit. Otherwise (a table symbol,
+    // or a non-zero table index from a multi-table module) it is printed
+    // first. A disassembler can also hand us an arbitrary immediate here when
+    // decoding misaligned bytes, so print it rather than asserting.
     OS << "\t";
     OS << getMnemonic(*MI).first;
     OS << " ";
-    if (MI->getOperand(TableOperand).isExpr()) {
+    const MCOperand &TableOp = MI->getOperand(TableOperand);
+    if (TableOp.isExpr() || (TableOp.isImm() && TableOp.getImm() != 0)) {
       printOperand(MI, TableOperand, STI, OS);
       OS << ", ";
-    } else {
-      assert(MI->getOperand(TableOperand).getImm() == 0);
     }
     printOperand(MI, TypeOperand, STI, OS);
     if (MI->getOpcode() == WebAssembly::CALL_INDIRECT)
diff --git a/llvm/test/MC/Disassembler/WebAssembly/wasm.txt b/llvm/test/MC/Disassembler/WebAssembly/wasm.txt
index 3a418ea32c12a..e0f8598db71e6 100644
--- a/llvm/test/MC/Disassembler/WebAssembly/wasm.txt
+++ b/llvm/test/MC/Disassembler/WebAssembly/wasm.txt
@@ -20,6 +20,11 @@
 # FIXME: WebAssemblyInstPrinter does not print immediates.
 0x11 0x80 0x01 0x00
 
+# A non-zero table index (a multi-table module, or misaligned bytes fed to the
+# disassembler) must be printed rather than trigger an assertion.
+# CHECK: call_indirect 1, 0
+0x11 0x00 0x01
+
 # CHECK: call 0
 0x10 0x00
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/210120