[TableGen] Allow DAG isel patterns to override default operands.

When a Tablegen instruction description uses `OperandWithDefaultOps`,
isel patterns for that instruction don't have to fill in the default
value for the operand in question. But the flip side is that they
actually //can't// override the defaults even if they want to.

This will be very inconvenient for the Arm backend, when we start
wanting to write isel patterns that generate the many MVE predicated
vector instructions, in the form with predication actually enabled. So
this small Tablegen fix makes it possible to write an isel pattern
either with or without values for a defaulted operand, and have the
default values filled in only if they are not overridden.

If all the defaulted operands come at the end of the instruction's
operand list, there's a natural way to match them up to the arguments
supplied in the pattern: consume pattern arguments until you run out,
then fill in any missing instruction operands with their default
values. But if defaulted and non-defaulted operands are interleaved,
it's less clear what to do. This does happen in existing targets (the
first example I came across was KILLGT, in the AMDGPU/R600 backend),
and of course they expect the previous behaviour (that the default for
those operands is used and a pattern argument is not consumed), so for
backwards compatibility I've stuck with that.

Reviewers: nhaehnle, hfinkel, dmgreen

Subscribers: mehdi_amini, javed.absar, tpr, kristof.beyls, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63814

llvm-svn: 365114

1 files changed, 20 insertions, 6 deletions

diff --git a/llvm/utils/TableGen/CodeGenDAGPatterns.cpp b/llvm/utils/TableGen/CodeGenDAGPatterns.cpp
index 54f3e00..9f87b3d 100644
--- a/llvm/utils/TableGen/CodeGenDAGPatterns.cpp
+++ b/llvm/utils/TableGen/CodeGenDAGPatterns.cpp
@@ -2437,18 +2437,32 @@ bool TreePatternNode::ApplyTypeConstraints(TreePattern &TP, bool NotRegisters) {
       }
     }
 
+    // If one or more operands with a default value appear at the end of the
+    // formal operand list for an instruction, we allow them to be overridden
+    // by optional operands provided in the pattern.
+    //
+    // But if an operand B without a default appears at any point after an
+    // operand A with a default, then we don't allow A to be overridden,
+    // because there would be no way to specify whether the next operand in
+    // the pattern was intended to override A or skip it.
+    unsigned NonOverridableOperands = Inst.getNumOperands();
+    while (NonOverridableOperands > 0 &&
+           CDP.operandHasDefault(Inst.getOperand(NonOverridableOperands-1)))
+      --NonOverridableOperands;
+
     unsigned ChildNo = 0;
     for (unsigned i = 0, e = Inst.getNumOperands(); i != e; ++i) {
       Record *OperandNode = Inst.getOperand(i);
 
-      // If the instruction expects a predicate or optional def operand, we
-      // codegen this by setting the operand to it's default value if it has a
-      // non-empty DefaultOps field.
-      if (OperandNode->isSubClassOf("OperandWithDefaultOps") &&
-          !CDP.getDefaultOperand(OperandNode).DefaultOps.empty())
+      // If the operand has a default value, do we use it? We must use the
+      // default if we've run out of children of the pattern DAG to consume,
+      // or if the operand is followed by a non-defaulted one.
+      if (CDP.operandHasDefault(OperandNode) &&
+          (i < NonOverridableOperands || ChildNo >= getNumChildren()))
         continue;
 
-      // Verify that we didn't run out of provided operands.
+      // If we have run out of child nodes and there _isn't_ a default
+      // value we can use for the next operand, give an error.
       if (ChildNo >= getNumChildren()) {
         emitTooFewOperandsError(TP, getOperator()->getName(), getNumChildren());
         return false;