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For the trace recorder, use a wrapper function with the PreserveAll ABI. #293

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Merged
ltratt merged 1 commit into from
Dec 10, 2025
Merged

For the trace recorder, use a wrapper function with the PreserveAll ABI. #293

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1 change: 1 addition & 0 deletions llvm/include/llvm/Transforms/Yk/BasicBlockTracer.h
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@

// The name of the trace function - used in swt tracing.
#define YK_TRACE_FUNCTION "__yk_trace_basicblock"
#define YK_TRACE_FUNCTION_WRAPPER "__yk_trace_basicblock_wrapper"

// The name of the thread tracing state thread local.
#define YK_THREAD_TRACING_STATE_TL "__yk_thread_tracing_state"
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28 changes: 24 additions & 4 deletions llvm/lib/Transforms/Yk/BasicBlockTracer.cpp
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Expand Up @@ -75,9 +75,28 @@ struct YkBasicBlockTracer : public ModulePass {

FunctionType *FType = FunctionType::get(
ReturnType, {FunctionIndexArgType, BlockIndexArgType}, false);
Function *TraceFunc = Function::Create(
Function *TraceFuncInner = Function::Create(
FType, GlobalVariable::ExternalLinkage, YK_TRACE_FUNCTION, M);

// Define the trace recorder wrapper.
//
// We use a wrapper so we can use an ABI that disrupts register allocation
// in the interpreter code to a lesser extent. The wrapper is required
// because the actual recorder function is defined in the JIT runtime's
// shared object and we can't change the ABI for library calls.
llvm::Function *TraceFunc = Function::Create(
FType, llvm::Function::InternalLinkage, YK_TRACE_FUNCTION_WRAPPER, M);
TraceFunc->setCallingConv(llvm::CallingConv::PreserveAll);
TraceFunc->addFnAttr(YK_OUTLINE_FNATTR);
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@vext01 vext01 Dec 10, 2025

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As is often the case: raise a PR, spot a potential tweak.

I'm just wondering if I should mark the wrapper noinline...

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@ltratt ltratt Dec 10, 2025

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I don't think we mind if it's inlined: in a sense, we do the "right thing" no matter which stack frame we run things in.

[That said, my guess would be that LLVM won't inline functions with this ABI. But I could be wrong about that.]

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@vext01 vext01 Dec 10, 2025

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OK, shall we try this as-is then?

llvm::BasicBlock *WrapperBB = BasicBlock::Create(Context, "", TraceFunc);
IRBuilder<> Builder(WrapperBB);
std::vector<Value *> InnerArgs;
for (Argument &A : TraceFunc->args()) {
InnerArgs.push_back(&A);
}
Builder.CreateCall(TraceFuncInner, InnerArgs);
Builder.CreateRetVoid();

// Metadata used to help the serialiser identify the purpose of a block.
//
// This block is a "are we tracing" check:
Expand All @@ -90,7 +109,6 @@ struct YkBasicBlockTracer : public ModulePass {
MDNode *SerialiseBBMD =
MDNode::get(Context, MDString::get(Context, "swt-serialise-bb"));

IRBuilder<> Builder(Context);
uint32_t FunctionIndex = 0;
for (auto &F : M) {
// If we won't ever trace this, don't insert calls to the tracer, as it
Expand Down Expand Up @@ -160,8 +178,10 @@ struct YkBasicBlockTracer : public ModulePass {
// Make the block that calls the recorder.
BasicBlock *RecBB = llvm::BasicBlock::Create(Context, "", &F, RestBB);
Builder.SetInsertPoint(RecBB);
Builder.CreateCall(TraceFunc, {Builder.getInt32(FunctionIndex),
Builder.getInt32(BlockIndex)});
CallInst *CI =
Builder.CreateCall(TraceFunc, {Builder.getInt32(FunctionIndex),
Builder.getInt32(BlockIndex)});
CI->setCallingConv(llvm::CallingConv::PreserveAll);
Builder.CreateBr(RestBB);

// Update the terminator of the "are we tracing?" block We jump over
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18 changes: 9 additions & 9 deletions llvm/test/Transforms/Yk/BasicBlockTracer.ll
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@@ -1,7 +1,7 @@
; RUNNING TEST EXAMPLE: llvm-lit llvm/test/Transforms/Yk/BasicBlockTracer.ll
; RUN: llc -stop-after yk-basicblock-tracer-pass --yk-basicblock-tracer < %s | FileCheck %s

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 0)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 0)
define dso_local noundef i32 @main() #0 {
%1 = alloca i32, align 4
%2 = alloca i32, align 4
Expand All @@ -11,47 +11,47 @@ define dso_local noundef i32 @main() #0 {
store i32 0, i32* %3, align 4
br label %4

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 1)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 1)
4: ; preds = %13, %0
%5 = load i32, i32* %3, align 4
%6 = icmp slt i32 %5, 43
br i1 %6, label %7, label %16

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 2)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 2)
7: ; preds = %4
%8 = load i32, i32* %3, align 4
%9 = icmp eq i32 %8, 42
br i1 %9, label %10, label %12

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 3)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 3)
10: ; preds = %7
%11 = load i32, i32* %3, align 4
store i32 %11, i32* %1, align 4
br label %17

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 4)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 4)
12: ; preds = %7
br label %13

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 5)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 5)
13: ; preds = %12
%14 = load i32, i32* %3, align 4
%15 = add nsw i32 %14, 1
store i32 %15, i32* %3, align 4
br label %4, !llvm.loop !6

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 6)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 6)
16: ; preds = %4
store i32 0, i32* %1, align 4
br label %17

; CHECK: call void @__yk_trace_basicblock(i32 0, i32 7)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 0, i32 7)
17: ; preds = %16, %10
%18 = load i32, i32* %1, align 4
ret i32 %18
}

; CHECK: call void @__yk_trace_basicblock(i32 1, i32 0)
; CHECK: call preserve_allcc void @__yk_trace_basicblock_wrapper(i32 1, i32 0)
define dso_local noundef i32 @_Z5checki(i32 noundef %0) #1 {
%2 = alloca i32, align 4
store i32 %0, i32* %2, align 4
Expand Down