[ValueTracking] Imply poison if ValAssumePoison is identical to V

[nikic]

What is the motivation for this? Generally speaking, we leave this to CSE/GVN and don't try to handle identical instructions.

[StephenFan]

In D149935#4321253, @nikic wrote:

What is the motivation for this? Generally speaking, we leave this to CSE/GVN and don't try to handle identical instructions.

There are some regressions in D149404. I want to solve some of these regressions in that patch. For example, test auto_gen_5_logical_fmf in and-fcmp.ll.
And I found that multiple instcombines would be executed before GVN. I am concerned that the regressions in D149404 would make some redundancies no longer be optimized by GVN. But I haven't looked in depth yet.

llvm/lib/Analysis/ValueTracking.cpp

return ::canCreateUndefOrPoison(Op, /*PoisonOnly=*/true,
                                   ConsiderFlagsAndMetadata);
 }
 
 static bool directlyImpliesPoison(const Value *ValAssumedPoison,
                                   const Value *V, unsigned Depth) {
   if (ValAssumedPoison == V)
     return true;
 
+  auto *IAssumedPoison = dyn_cast<Instruction>(ValAssumedPoison);
+  auto *IV = dyn_cast<Instruction>(V);
+  if (IAssumedPoison && IV && IAssumedPoison->isIdenticalTo(IV))
+    return true;
+
   const unsigned MaxDepth = 2;
   if (Depth >= MaxDepth)
     return false;
 
   if (const auto *I = dyn_cast<Instruction>(V)) {
     if (any_of(I->operands(), [=](const Use &Op) {
           return propagatesPoison(Op) &&
                  directlyImpliesPoison(ValAssumedPoison, Op, Depth + 1);

llvm/test/Transforms/InstCombine/signed-truncation-check.ll

 ; CHECK-NEXT:    [[T2:%.*]] = icmp sgt i8 [[T1]], -1
 ; CHECK-NEXT:    call void @use1(i1 [[T2]])
 ; CHECK-NEXT:    [[T3:%.*]] = trunc i32 [[ARG]] to i8
 ; CHECK-NEXT:    call void @use8(i8 [[T3]])
 ; CHECK-NEXT:    [[T4:%.*]] = sext i8 [[T3]] to i32
 ; CHECK-NEXT:    call void @use32(i32 [[T4]])
 ; CHECK-NEXT:    [[T5:%.*]] = icmp eq i32 [[T4]], [[ARG]]
 ; CHECK-NEXT:    call void @use1(i1 [[T5]])
-; CHECK-NEXT:    [[T6:%.*]] = select i1 [[T2]], i1 [[T5]], i1 false
+; CHECK-NEXT:    [[T6:%.*]] = and i1 [[T2]], [[T5]]
 ; CHECK-NEXT:    ret i1 [[T6]]
 ;
   %t1 = trunc i32 %arg to i8
   call void @use8(i8 %t1)
   %t2 = icmp sgt i8 %t1, -1
   call void @use1(i1 %t2)
   %t3 = trunc i32 %arg to i8
   call void @use8(i8 %t3)