Use the frame index side table for byval and inalloca arguments

Summary:
For inalloca functions, this is a very common code pattern:

  %argpack = type <{ i32, i32, i32 }>
  define void @f(%argpack* inalloca %args) {
  entry:
    %a = getelementptr inbounds %argpack, %argpack* %args, i32 0, i32 0
    %b = getelementptr inbounds %argpack, %argpack* %args, i32 0, i32 1
    %c = getelementptr inbounds %argpack, %argpack* %args, i32 0, i32 2
    tail call void @llvm.dbg.declare(metadata i32* %a, ... "a")
    tail call void @llvm.dbg.declare(metadata i32* %c, ... "b")
    tail call void @llvm.dbg.declare(metadata i32* %b, ... "c")

Even though these GEPs can be simplified to a constant offset from EBP
or RSP, we don't do that at -O0, and each GEP is computed into a
register. Registers used to compute argument addresses are typically
spilled and clobbered very quickly after the initial computation, so
live debug variable tracking loses information very quickly if we use
DBG_VALUE instructions.

This change moves processing of dbg.declare between argument lowering
and basic block isel, so that we can ask if an argument has a frame
index or not. If the argument lives in a register as is the case for
byval arguments on some targets, then we don't put it in the side table
and during ISel we emit DBG_VALUE instructions.

Reviewers: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32980

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302483 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 9 insertions, 9 deletions

diff --git a/lib/CodeGen/SelectionDAG/FastISel.cpp b/lib/CodeGen/SelectionDAG/FastISel.cpp
index 8c98e3740f6..f37790d0983 100644
--- a/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1150,16 +1150,16 @@ bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
       return true;
     }
 
-    unsigned Offset = 0;
+    // Byval arguments with frame indices were already handled after argument
+    // lowering and before isel.
+    const auto *Arg =
+        dyn_cast<Argument>(Address->stripInBoundsConstantOffsets());
+    if (Arg && FuncInfo.getArgumentFrameIndex(Arg) != INT_MAX)
+      return true;
+
     Optional<MachineOperand> Op;
-    if (const auto *Arg = dyn_cast<Argument>(Address))
-      // Some arguments' frame index is recorded during argument lowering.
-      Offset = FuncInfo.getArgumentFrameIndex(Arg);
-    if (Offset)
-      Op = MachineOperand::CreateFI(Offset);
-    if (!Op)
-      if (unsigned Reg = lookUpRegForValue(Address))
-        Op = MachineOperand::CreateReg(Reg, false);
+    if (unsigned Reg = lookUpRegForValue(Address))
+      Op = MachineOperand::CreateReg(Reg, false);
 
     // If we have a VLA that has a "use" in a metadata node that's then used
     // here but it has no other uses, then we have a problem. E.g.,