| Issue |
179127
|
| Summary |
[DAG] Add isKnownNonZero to enable optimizations
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
fbrv
|
DAGCombiner has optimizations that depend on proving values are non-zero but cannot easily do so, forcing conservative defaults.
### Examples
`BuildLogBase2` in DAGCombiner.cpp has a `KnownNonZero` parameter:
```cpp
/// Determines the LogBase2 value for a non-null input value using the
/// transform: LogBase2(V) = (EltBits - 1) - ctlz(V).
SDValue DAGCombiner::BuildLogBase2(SDValue V, const SDLoc &DL,
bool KnownNonZero, bool InexpensiveOnly,
```
Declaration defaults to `false`:
```cpp
SDValue BuildLogBase2(SDValue V, const SDLoc &DL,
bool KnownNeverZero = false,
```
**All call sites use the default** - none pass `true` because there's no way to prove a value is non-zero:
```cpp
if (SDValue LogBase2 = BuildLogBase2(N1, DL)) { // Uses default false
```
Similarly, `takeInexpensiveLog2` has an `AssumeNonZero` flag that resorts to checking instruction flags as a workaround:
```cpp
if (AssumeNonZero || Op->getFlags().hasNoUnsignedWrap() ||
Op->getFlags().hasNoSignedWrap() || isOneConstant(Op.getOperand(0)))
```
### Proposed Solution
Add to SelectionDAG:
```cpp
bool SelectionDAG::isKnownNonZero(SDValue Op, unsigned Depth = 0) const;
```
This should enable:
- `BuildLogBase2` calls with `KnownNonZero = true` when provable
- Simpler non-zero checks without flag inspection
- Match IR's `llvm::isKnownNonZero()` capabilities
---
If makes sense I can create a POC patch
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