[llvm] [AMDGPU] Assert if stack grows downwards. (PR #119888)

[Sergei Barannikov via llvm-commits]

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@@ -4041,17 +4041,15 @@ SDValue SITargetLowering::lowerDYNAMIC_STACKALLOCImpl(SDValue Op,
   Chain = SP.getValue(1);
   MaybeAlign Alignment = cast<ConstantSDNode>(Tmp3)->getMaybeAlignValue();
   const TargetFrameLowering *TFL = Subtarget->getFrameLowering();
-  unsigned Opc =
-      TFL->getStackGrowthDirection() == TargetFrameLowering::StackGrowsUp
-          ? ISD::ADD
-          : ISD::SUB;
+  assert(TFL->getStackGrowthDirection() == TargetFrameLowering::StackGrowsUp &&
+         "Stack grows upwards for AMDGPU");
 
   SDValue ScaledSize = DAG.getNode(
       ISD::SHL, dl, VT, Size,
       DAG.getConstant(Subtarget->getWavefrontSizeLog2(), dl, MVT::i32));
 
   Align StackAlign = TFL->getStackAlign();
-  Tmp1 = DAG.getNode(Opc, dl, VT, SP, ScaledSize); // Value
+  Tmp1 = DAG.getNode(ISD::ADD, dl, VT, SP, ScaledSize); // Value
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s-barannikov wrote:

(not related to this PR, but I thought it was worth reporting)

If I'm reading correctly, the logic is broken for the case when the object alignment is greater than the stack alignment.
The `ISD::AND` below may result in an address that points below the current SP, possibly into a previously allocated object.

Consider: `SP = 40, StackAlign = 8, ScaledSize = 16, (Scaled)Alignment = 32`
```
Tmp1 = ADD 40, 16 = 56
Tmp1 = AND 56, -32 = 32
```

The new address (32) is less than the previous value of the stack pointer (40).


https://github.com/llvm/llvm-project/pull/119888