Add alloca instruction to Subzero bitcode reader.

src/PNaClTranslator.cpp

Index: src/PNaClTranslator.cpp
diff --git a/src/PNaClTranslator.cpp b/src/PNaClTranslator.cpp
index f315294b21dd4a53f5096017aaefa5ce3a765716..79759016fee089844f6b3178b165a3c0cea84f0e 100644
--- a/src/PNaClTranslator.cpp
+++ b/src/PNaClTranslator.cpp
@@ -1510,6 +1510,22 @@ void FunctionParser::ProcessRecord() {
     InstIsTerminating = true;
     break;
   }
+  case naclbitc::FUNC_CODE_INST_ALLOCA: {
+    // ALLOCA: [Size, align]
+    if (!isValidRecordSize(2, "function block alloca")) return;

[jvoung (off chromium), 2014/09/05 16:55:04]

return on separate line

[Karl, 2014/09/08 21:16:23]

Done.

+    Ice::Operand *ByteCount = getRelativeOperand(Values[0]);
+    if (ByteCount->getType() != Ice::IceType_i32) {
+      std::string Buffer;
+      raw_string_ostream StrBuf(Buffer);
+      StrBuf << "Alloca on non-i32 value. Found: " << ByteCount;
+      Error(StrBuf.str());
+      return;
+    }
+    unsigned Align = static_cast<unsigned>(Values[1]);

[jvoung (off chromium), 2014/09/05 16:55:04]

Should probably check that Align <= 29 or so? I think the max that LLVM supports
(for arbitrary reasons) is 2 ** 29:
http://llvm.org/docs/LangRef.html#alloca-instruction

You don't want to shift "1 << 33" below, for example.

[Karl, 2014/09/08 21:16:23]

Good point.

+    Ice::Variable *Dest = NextInstVar(Context->getIcePointerType());
+    Inst = Ice::InstAlloca::create(Func, ByteCount, (1 << Align) >> 1, Dest);

[Jim Stichnoth, 2014/09/05 23:03:39]

Any reason why it isn't (1<<(Align-1)) as opposed to ((1<<Align)>>1)?

What happens when Align==0?  (This would be the major difference between the two
expression, I think.)

The LLVM alloca documentation doesn't say that the alignment must be a power of
2.  Is that a PNaCl simplification, or is the LLVM documentation wrong?

[Karl, 2014/09/08 21:16:23]

In LLVM and PNaCl, 0 is allowed, and hence the special calculation.
Also, while LLVM isn't explicit about the power of 2 for alloca, it is
elsewhere. In addition, the LLVM bitcode writer explicitly assumes this and
converts it to the nearest enclosing power of 2. I will make sure that we make
this explicit in the PNaCl documentation.

+    break;
+  }
   default:
     // Generate error message!
     BlockParserBaseClass::ProcessRecord();