Skip to content

[AMDGPU] Assert if stack grows downwards. #119888

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 2 commits into from
Dec 14, 2024
Merged

[AMDGPU] Assert if stack grows downwards. #119888

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
ab5ed10
assert if stack grows upwards
easyonaadit Dec 13, 2024
f7f11a8
review comments
easyonaadit Dec 14, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
4 changes: 2 additions & 2 deletions llvm/lib/Target/AMDGPU/AMDGPURegisterBankInfo.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1181,8 +1181,8 @@ bool AMDGPURegisterBankInfo::applyMappingDynStackAlloc(

// Guard in case the stack growth direction ever changes with scratch
// instructions.
if (TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown)
return false;
assert(TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsUp &&
"Stack grows upwards for AMDGPU");

Register Dst = MI.getOperand(0).getReg();
Register AllocSize = MI.getOperand(1).getReg();
Expand Down
8 changes: 3 additions & 5 deletions llvm/lib/Target/AMDGPU/SIISelLowering.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -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
Copy link
Contributor

@s-barannikov s-barannikov Dec 14, 2024 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

(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).

ADD
And the returned address (Tmp1) points past the allocated object. Probably I don't understand something...

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Hi,
There are 2 things to consider here:

  1. For AMDGPU, the Stack grows upwards (low addr to high addr)
  2. The code here is based on the default implementation, which itself doesn't seem to adjust for a growing up stack.

This patch is actually just a part of fixing the logic flaw you pointed out. There is a fix in progress, you could follow it here:
#119822

Copy link
Contributor

@s-barannikov s-barannikov Dec 16, 2024 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The default implementation is dead code for StackGrowsUp. I was going to fix it, but I couldn't add tests for the changes.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Other than scaling for wave size, the code changes in #119822 serve as a generic implementation for a growing up stack. And there are test cases along with it as well. Could this be replicated?

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The only other target that uses positive stack growth is NVPTX

if (Alignment && *Alignment > StackAlign) {
Tmp1 = DAG.getNode(
ISD::AND, dl, VT, Tmp1,
Expand Down
Loading