[PATCH] D29872: Do not legalize large add with addc/adde, introduce addcarry and do it with uaddo/addcarry

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

I haven't looked at carry codegen enough to approve, but see inline comments for some nits.



================
Comment at: include/llvm/CodeGen/ISDOpcodes.h:215-218
     /// Carry-setting nodes for multiple precision addition and subtraction.
     /// These nodes take two operands of the same value type, and produce two
     /// results.  The first result is the normal add or sub result, the second
     /// result is the carry flag result.
----------------
If I'd just stumbled into this code, I'd be confused why we have ADDC and ADDE and ADDCARRY. If the intent is to remove these, please add a 'FIXME' comment here that says we should get rid of these.


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:2084-2086
+  // canonicalize constant to RHS
+  ConstantSDNode *N0C = dyn_cast<ConstantSDNode>(N0);
+  ConstantSDNode *N1C = dyn_cast<ConstantSDNode>(N1);
----------------
This is copied from above, but comments are supposed to be full sentences (capital letter, period).
Use 'auto *' with dyn_cast because the type is obvious from the code and variable name.


================
Comment at: lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp:517
+  SDValue Ops[3] = { N->getOperand(0), N->getOperand(1) };
+  unsigned C = N->getNumOperands();
+  assert(C <= 3 && "Too many operands");
----------------
I'd call this 'NumOps' or similar. 'C' in LLVM usually means some kind of constant value.


================
Comment at: lib/CodeGen/TargetLoweringBase.cpp:913-914
 
+    // ADDCARRY and SUBCARRY are new additions which needs
+    // to default to expand to not break all backends.
+    setOperationAction(ISD::ADDCARRY, VT, Expand);
----------------
This code comment won't age well. Delete.


================
Comment at: lib/Target/X86/X86ISelLowering.cpp:34407
+// Optimize RES, EFLAGS = X86ISD::ADD LHS, RHS
+static SDValue combineADD(SDNode *N, SelectionDAG &DAG,
+                          X86TargetLowering::DAGCombinerInfo &DCI) {
----------------
RKSimon wrote:
> We already have a combineAdd function - this should be named something like combineX86Add
+1 - we definitely do not want function names that only differ in captitalization.


================
Comment at: lib/Target/X86/X86ISelLowering.cpp:34507
+  if (isAllOnesConstant(N->getOperand(1)) && N->hasAnyUseOfValue(1)) {
+    auto Carry = N->getOperand(0);
+    while (Carry.getOpcode() == ISD::TRUNCATE ||
----------------
auto -> SDValue


================
Comment at: lib/Target/X86/X86ISelLowering.cpp:34519
+        Carry.getOpcode() == X86ISD::SETCC_CARRY) {
+      auto Cond = cast<ConstantSDNode>(Carry.getOperand(0));
+      if (Cond->getZExtValue() == X86::COND_B)
----------------
auto -> auto *


https://reviews.llvm.org/D29872