Fix for false dependency identification (pr31143) - reading undef values shouldn't be considers as a use

[myatsina]

Summary

This commit fixes pr31143 (https://llvm.org/bugs/show_bug.cgi?id=31143).

ExecutionDepsFix pass couldn't identify the partial register update of ROUNDSSr:
%XMM1<def,tied1> = ROUNDSSr %XMM1<undef,tied0>, %XMM0, 9

The root cause of this was because ROUNDSSr instruction is marked as reading %XMM1, but it's actually reading an undef value.
This patch fixes MachineInstruction - now a use of an undef value isn't considered a real use anymore.

[mkuper]

What we do for the other instructions is simply not have the additional source register (and use the _Int variants for when we need it, which is currently only for intrinsics).
I've posted a fix in that vein as D27323. We can revisit that decision, but that (and a refactoring of the TD files) ought to happen for all instructions, I wouldn't want to leave ROUND as a special case.

Having said that, if there's consensus to take this instead of D27323, I won't stand in the way.

lib/CodeGen/MachineInstr.cpp

 
 /// findRegisterUseOperandIdx() - Returns the MachineOperand that is a use of
 /// the specific register or -1 if it is not found. It further tightens
 /// the search criteria to a use that kills the register if isKill is true.
 int MachineInstr::findRegisterUseOperandIdx(
     unsigned Reg, bool isKill, const TargetRegisterInfo *TRI) const {
   for (unsigned i = 0, e = getNumOperands(); i != e; ++i) {
     const MachineOperand &MO = getOperand(i);
-    if (!MO.isReg() || !MO.isUse())
+    if (!MO.isReg() || !MO.isUse() || MO.isUndef())
       continue;
     unsigned MOReg = MO.getReg();
     if (!MOReg)
       continue;
     if (MOReg == Reg || (TRI && TargetRegisterInfo::isPhysicalRegister(MOReg) &&
                          TargetRegisterInfo::isPhysicalRegister(Reg) &&
                          TRI->isSubRegister(MOReg, Reg)))
       if (!isKill || MO.isKill())

test/CodeGen/X86/pr31143.ll

+; RUN: llc --mattr=+sse4.1 %s -o - | FileCheck %s
+
+target triple = "x86_64-pc-linux-gnu"
+
+
+; CHECK:      xorps   [[XMM1:%xmm[0-9]+]], [[XMM1]]
+; CHECK-NEXT: roundss $9, [[XMM0:%xmm[0-9]+]], [[XMM1]]
+
+define void @test(float* nocapture %a, <4 x float>* nocapture %b, i32 %k) {
+entry:
+  br label %for.body
+
+for.body:
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
+  %v = load float, float* %arrayidx, align 4
+  %floor = call float @floorf(float %v)
+  %sub = fsub float %floor, %v
+  %v1 = insertelement <4 x float> undef, float %sub, i32 0
+  %br = shufflevector <4 x float> %v1, <4 x float> undef, <4 x i32> <i32 0, i32 0, i32 0, i32 0>
+  store volatile <4 x float> %br, <4 x float>* %b, align 4
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %k
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret void
+}
+
+declare float @floorf(float) nounwind readnone