[IR] Add zext nneg flag (#67982)

Add an nneg flag to the zext instruction, which specifies that the
argument is non-negative. Otherwise, the result is a poison value.

The primary use-case for the flag is to preserve information when sext
gets replaced with zext due to range-based canonicalization. The nneg
flag allows us to convert the zext back into an sext later. This is
useful for some optimizations (e.g. a signed icmp can fold with sext but
not zext), as well as some targets (e.g. RISCV prefers sext over zext).

Discourse thread: https://discourse.llvm.org/t/rfc-add-zext-nneg-flag/73914

This patch is based on https://reviews.llvm.org/D156444 by
@Panagiotis156, with some implementation simplifications and additional
tests.

---------

Co-authored-by: Panagiotis K <karouzakispan@gmail.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 28addb9..fcba736 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -4877,12 +4877,13 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
       Value *Op;
       unsigned OpTypeID;
       if (getValueTypePair(Record, OpNum, NextValueNo, Op, OpTypeID, CurBB) ||
-          OpNum+2 != Record.size())
+          OpNum + 1 > Record.size())
         return error("Invalid record");
 
-      ResTypeID = Record[OpNum];
+      ResTypeID = Record[OpNum++];
       Type *ResTy = getTypeByID(ResTypeID);
-      int Opc = getDecodedCastOpcode(Record[OpNum + 1]);
+      int Opc = getDecodedCastOpcode(Record[OpNum++]);
+
       if (Opc == -1 || !ResTy)
         return error("Invalid record");
       Instruction *Temp = nullptr;
@@ -4898,6 +4899,9 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
           return error("Invalid cast");
         I = CastInst::Create(CastOp, Op, ResTy);
       }
+      if (OpNum < Record.size() && isa<PossiblyNonNegInst>(I) &&
+          (Record[OpNum] & (1 << bitc::PNNI_NON_NEG)))
+        I->setNonNeg(true);
       InstructionList.push_back(I);
       break;
     }