| Issue |
138513
|
| Summary |
DAGCombiner/SimplifyDemandedBits making ISel DAG more poisonous for bitcasted vector
|
| Labels |
llvm:SelectionDAG
|
| Assignees |
bjope
|
| Reporter |
bjope
|
Consider IR in this example:
```
define void @bar(i16 %a, ptr %p) {
entry:
%.upto4 = insertelement <8 x i16> <i16 0, i16 1, i16 2, i16 3, i16 poison, i16 poison, i16 poison, i16 poison>, i16 %a, i64 4
%.upto5 = insertelement <8 x i16> %.upto4, i16 5, i64 5
%.upto6 = insertelement <8 x i16> %.upto5, i16 6, i64 6
%.upto7 = insertelement <8 x i16> %.upto6, i16 7, i64 7
%bitcast = bitcast <8 x i16> %.upto7 to i128
%lshr = lshr i128 %bitcast, 48
%trunc = trunc i128 %lshr to i32
store i32 %trunc, ptr %p
ret void
}
```
When running `llc -debug ...` on the above we see this:
```
Initial selection DAG: %bb.0 'bar:entry'
SelectionDAG has 33 nodes:
t0: ch,glue = EntryToken
t24: i128 = Constant<48>
t28: i64 = Constant<0>
t11: v8i16 = BUILD_VECTOR Constant:i16<0>, Constant:i16<1>, Constant:i16<2>, Constant:i16<3>, poison:i16, poison:i16, poison:i16, poison:i16
t2: i32,ch = CopyFromReg t0, Register:i32 %0
t3: i16 = truncate t2
t13: v8i16 = insert_vector_elt t11, t3, Constant:i64<4>
t16: v8i16 = insert_vector_elt t13, Constant:i16<5>, Constant:i64<5>
t19: v8i16 = insert_vector_elt t16, Constant:i16<6>, Constant:i64<6>
t22: v8i16 = insert_vector_elt t19, Constant:i16<7>, Constant:i64<7>
t23: i128 = bitcast t22
t26: i128 = srl t23, Constant:i8<48>
t27: i32 = truncate t26
t5: i64,ch = CopyFromReg t0, Register:i64 %1
t30: ch = store<(store (s32) into %ir.p)> t0, t27, t5, undef:i64
t32: ch = X86ISD::RET_GLUE t30, TargetConstant:i32<0>
...
Combining: t27: i32 = truncate t26
Replacing.2 t22: v8i16 = insert_vector_elt t19, Constant:i16<7>, Constant:i64<7>
With: t19: v8i16 = insert_vector_elt t16, Constant:i16<6>, Constant:i64<6>
Combining: t27: i32 = truncate t26
Replacing.2 t19: v8i16 = insert_vector_elt t16, Constant:i16<6>, Constant:i64<6>
With: t16: v8i16 = insert_vector_elt t13, Constant:i16<5>, Constant:i64<5>
Combining: t27: i32 = truncate t26
Replacing.2 t16: v8i16 = insert_vector_elt t13, Constant:i16<5>, Constant:i64<5>
With: t13: v8i16 = insert_vector_elt t11, t3, Constant:i64<4>
...
Optimized lowered selection DAG: %bb.0 'bar:entry'
SelectionDAG has 18 nodes:
t0: ch,glue = EntryToken
t2: i32,ch = CopyFromReg t0, Register:i32 %0
t3: i16 = truncate t2
t35: v8i16 = BUILD_VECTOR undef:i16, undef:i16, undef:i16, Constant:i16<3>, t3, poison:i16, poison:i16, poison:i16
t23: i128 = bitcast t35
t26: i128 = srl t23, Constant:i8<48>
t27: i32 = truncate t26
t5: i64,ch = CopyFromReg t0, Register:i64 %1
t30: ch = store<(store (s32) into %ir.p)> t0, t27, t5, undef:i64
t32: ch = X86ISD::RET_GLUE t30, TargetConstant:i32<0>
...
```
The above looks wrong!
Before the DAG combine rewrites the bitcast is casting a vector without poisoned elements into an i128, but in the optimized selection DAG the bitcast is casting a vector with poisoned elements into i128 making the result poison. So the result of the bitcast is now posion.
I suspect TargetLowering::SimplifyDemandedBits/SimplifyDemandedVectorElts somehow is to blame for this.
When dealing with BITCAST in SimplifyDemandedBits it may call SimplifyDemandedVectorElts. We need to make sure that all smaller source elements mapping to a larger element are demanded when looking at a bitcast from 'small element' src vector to a 'large element' vector.
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