[X86] Enable forwarding bool arguments in tail calls (PR26305)

[hans]

Summary

The code was previously not able to trace a boolean argument at a call site back to the formal argument of the caller.

[rnk]

lib/Target/X86/X86ISelLowering.cpp

3666

Maybe a do { ... } while (false) loop is appropriate here?

test/CodeGen/X86/tail-call-bools.ll

1 ↗

(On Diff #49559)

Can you also run this in x86_64? The f_i32 guy will have to do movss %rcx, %xmm0 or something.

Also, maybe tail-call-casts.ll is a better name for the test, since it's not all about bools.

[hans]

lib/Target/X86/X86ISelLowering.cpp

3666

But then continue won't work?

test/CodeGen/X86/tail-call-bools.ll

1 ↗

(On Diff #49559)

My change doesn't affect tail call arguments passed in registers, so is mostly not relevant for x86_64.

I do want to look into x86_64 in a follow-up though. For bools, we currently generate:

movzbl  %dil, %edi
jmp     g_bool

i.e. we do make the tail call, but we don't recognize that the argument can be forwarded as-is. The float bitcast test will come in handy to check that we don't mess that up.

I've renamed the test case.

[hans]

Rename the test case.

[rnk]

lgtm

lib/Target/X86/X86ISelLowering.cpp

 
 /// Return true if the given stack call argument is already available in the
 /// same position (relatively) of the caller's incoming argument stack.
 static
 bool MatchingStackOffset(SDValue Arg, unsigned Offset, ISD::ArgFlagsTy Flags,
                          MachineFrameInfo *MFI, const MachineRegisterInfo *MRI,
                          const X86InstrInfo *TII, const CCValAssign &VA) {
   unsigned Bytes = Arg.getValueType().getSizeInBits() / 8;
+
+  for (;;) {
+    // Look through nodes that don't alter the bits of the incoming value.
+    unsigned Op = Arg.getOpcode();
+    if (Op == ISD::ZERO_EXTEND || Op == ISD::ANY_EXTEND || Op == ISD::BITCAST) {
+      Arg = Arg.getOperand(0);
+      continue;
+    }
+    if (Op == ISD::TRUNCATE) {
+      const SDValue &TruncInput = Arg.getOperand(0);
+      if (TruncInput.getOpcode() == ISD::AssertZext &&
+          cast<VTSDNode>(TruncInput.getOperand(1))->getVT() ==
+              Arg.getValueType()) {
+        Arg = TruncInput.getOperand(0);
+        continue;
+      }
+    }
+    break;
+  }
+
   int FI = INT_MAX;
   if (Arg.getOpcode() == ISD::CopyFromReg) {
     unsigned VR = cast<RegisterSDNode>(Arg.getOperand(1))->getReg();
     if (!TargetRegisterInfo::isVirtualRegister(VR))
       return false;
     MachineInstr *Def = MRI->getVRegDef(VR);
     if (!Def)
       return false;

test/CodeGen/X86/tail-call-casts.ll

+; RUN: llc -mtriple=i686-unknown-linux-gnu -o - %s | FileCheck %s
+
+declare void @g_bool(i1 zeroext)
+define void @f_bool(i1 zeroext %x) {
+entry:
+  tail call void @g_bool(i1 zeroext %x)
+  ret void
+
+; Forwarding a bool in a tail call works.
+; CHECK-LABEL: f_bool:
+; CHECK-NOT:   movz
+; CHECK:       jmp g_bool
+}
+
+
+declare void @g_float(float)
+define void @f_i32(i32 %x) {
+entry:
+  %0 = bitcast i32 %x to float
+  tail call void @g_float(float %0)
+  ret void
+
+; Forwarding a bitcasted value works too.
+; CHECK-LABEL: f_i32
+; CHECK-NOT:   mov
+; CHECK:       jmp g_float
+}