[RISCV] Fix a mistake in PostprocessISelDAG

[Luhaocong]

Summary

With the condition N->use_empty() , the root node of DAG always misses peephole optimization.
A dummy node is needed here.

[craig.topper]

Do you have test case?

[Luhaocong]

upload a test case

[Luhaocong]

In D119934#3326658, @craig.topper wrote:

Do you have test case?

store i64 %inc, i64* @g_8 is the root node of CurDAG，with N->use_empty() is true

[jrtc27]

llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll

183 ↗

(On Diff #409461)

This was already generated. If you use i32 loads/stores with @g_4 then both RV32 and RV64 see codegen differences. Using an i64 on RV32 gets a TokenFactor to glue together the splitting of the illegal i64 store into two legal i32 stores and so gives you a root node that's not one of the nodes you want to optimise.

194–195 ↗

(On Diff #409461)

Without this patch these two lines were:

addi    a0, a0, %lo(g_8)
addi    a1, a1, 1
sd      a1, 0(a0)

i.e. the %lo wasn't folded into the store's immediate due to the store being the root node

[craig.topper]

I suppose we inherited this bug from PowerPC. @nemanjai maybe you want to fix this for PowerPC?

[jrtc27]

llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll

198 ↗

(On Diff #409461)

Unless you feed the IR through opt, this isn't actually needed, you can just have entry's contents be if.then. The key thing is the ret isn't in the same basic block as the store as that would otherwise be the root for everything with a chain. Maybe it's a good idea to keep the dummy compare though so it's safe against optimisation silently folding the ret back into the basic block, as passing this through opt as it stands does nothing.

[jrtc27]

In D119934#3328022, @craig.topper wrote:

I suppose we inherited this bug from PowerPC. @nemanjai maybe you want to fix this for PowerPC?

X86 is the same from the looks of it. The only other implementation of PostprocessISelDAG is AMDGPU which does things a bit differently and doesn't seem to have an equivalent "check for uses".

[craig.topper]

In D119934#3328030, @jrtc27 wrote:

In D119934#3328022, @craig.topper wrote:

I suppose we inherited this bug from PowerPC. @nemanjai maybe you want to fix this for PowerPC?

X86 is the same from the looks of it. The only other implementation of PostprocessISelDAG is AMDGPU which does things a bit differently and doesn't seem to have an equivalent "check for uses".

With the current post-processing on X86 I don't thin you could get a failure. None of the opcodes that are being looked for have chain outputs so they can't be the root.

[jrtc27]

In D119934#3328037, @craig.topper wrote:

In D119934#3328030, @jrtc27 wrote:

In D119934#3328022, @craig.topper wrote:

I suppose we inherited this bug from PowerPC. @nemanjai maybe you want to fix this for PowerPC?

X86 is the same from the looks of it. The only other implementation of PostprocessISelDAG is AMDGPU which does things a bit differently and doesn't seem to have an equivalent "check for uses".

With the current post-processing on X86 I don't thin you could get a failure. None of the opcodes that are being looked for have chain outputs so they can't be the root.

Ok, so a possible bug waiting to happen but probably not currently an issue.

[Luhaocong]

update test case

[Luhaocong]

ping...

[craig.topper]

LGTM

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

for (SelectionDAG::allnodes_iterator I = CurDAG->allnodes_begin(),
     // Now that we did that, the node is dead.  Increment the iterator to the
     // next node to process, then delete N.
     ++I;
     CurDAG->DeleteNode(N);
   }
 }
 
 void RISCVDAGToDAGISel::PostprocessISelDAG() {
+  HandleSDNode Dummy(CurDAG->getRoot());
   SelectionDAG::allnodes_iterator Position = CurDAG->allnodes_end();
 
   bool MadeChange = false;
   while (Position != CurDAG->allnodes_begin()) {
     SDNode *N = &*--Position;
     // Skip dead nodes and any non-machine opcodes.
     if (N->use_empty() || !N->isMachineOpcode())
       continue;
 
     MadeChange |= doPeepholeSExtW(N);
     MadeChange |= doPeepholeLoadStoreADDI(N);
     MadeChange |= doPeepholeMaskedRVV(N);
   }
 
+  CurDAG->setRoot(Dummy.getValue());
+
   if (MadeChange)
     CurDAG->RemoveDeadNodes();
 }
 
 static SDNode *selectImmWithConstantPool(SelectionDAG *CurDAG, const SDLoc &DL,
                                          const MVT VT, int64_t Imm,
                                          const RISCVSubtarget &Subtarget) {
   assert(VT == MVT::i64 && "Expecting MVT::i64");