[IR] allow fast-math-flags on phi of FP values (2nd try)

The changes here are based on the corresponding diffs for allowing FMF on 'select':
D61917 <https://reviews.llvm.org/D61917>

As discussed there, we want to have fast-math-flags be a property of an FP value
because the alternative (having them on things like fcmp) leads to logical
inconsistency such as:
https://bugs.llvm.org/show_bug.cgi?id=38086

The earlier patch for select made almost no practical difference because most
unoptimized conditional code begins life as a phi (based on what I see in clang).
Similarly, I don't expect this patch to do much on its own either because
SimplifyCFG promptly drops the flags when converting to select on a minimal
example like:
https://bugs.llvm.org/show_bug.cgi?id=39535

But once we have this plumbing in place, we should be able to wire up the FMF
propagation and start solving cases like that.

The change to RecurrenceDescriptor::AddReductionVar() is required to prevent a
regression in a LoopVectorize test. We are intersecting the FMF of any
FPMathOperator there, so if a phi is not properly annotated, new math
instructions may not be either. Once we fix the propagation in SimplifyCFG, it
may be safe to remove that hack.

Differential Revision: https://reviews.llvm.org/D67564

llvm-svn: 372878

1 files changed, 23 insertions, 6 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 89a5b7b..9c37755 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -4629,31 +4629,48 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
       InstructionList.push_back(I);
       break;
     case bitc::FUNC_CODE_INST_PHI: { // PHI: [ty, val0,bb0, ...]
-      if (Record.size() < 1 || ((Record.size()-1)&1))
+      if (Record.size() < 1)
         return error("Invalid record");
+      // The first record specifies the type.
       FullTy = getFullyStructuredTypeByID(Record[0]);
       Type *Ty = flattenPointerTypes(FullTy);
       if (!Ty)
         return error("Invalid record");
 
-      PHINode *PN = PHINode::Create(Ty, (Record.size()-1)/2);
+      // Phi arguments are pairs of records of [value, basic block].
+      // There is an optional final record for fast-math-flags if this phi has a
+      // floating-point type.
+      size_t NumArgs = (Record.size() - 1) / 2;
+      if ((Record.size() - 1) % 2 == 1 && !Ty->isFloatingPointTy())
+        return error("Invalid record");
+
+      PHINode *PN = PHINode::Create(Ty, NumArgs);
       InstructionList.push_back(PN);
 
-      for (unsigned i = 0, e = Record.size()-1; i != e; i += 2) {
+      for (unsigned i = 0; i != NumArgs; i++) {
         Value *V;
         // With the new function encoding, it is possible that operands have
         // negative IDs (for forward references).  Use a signed VBR
         // representation to keep the encoding small.
         if (UseRelativeIDs)
-          V = getValueSigned(Record, 1+i, NextValueNo, Ty);
+          V = getValueSigned(Record, i * 2 + 1, NextValueNo, Ty);
         else
-          V = getValue(Record, 1+i, NextValueNo, Ty);
-        BasicBlock *BB = getBasicBlock(Record[2+i]);
+          V = getValue(Record, i * 2 + 1, NextValueNo, Ty);
+        BasicBlock *BB = getBasicBlock(Record[i * 2 + 2]);
         if (!V || !BB)
           return error("Invalid record");
         PN->addIncoming(V, BB);
       }
       I = PN;
+
+      // If there are an even number of records, the final record must be FMF.
+      if (Record.size() % 2 == 0) {
+        assert(isa<FPMathOperator>(I) && "Unexpected phi type");
+        FastMathFlags FMF = getDecodedFastMathFlags(Record[Record.size() - 1]);
+        if (FMF.any())
+          I->setFastMathFlags(FMF);
+      }
+
       break;
     }