[llvm] [SystemZ] Add proper mcount handling (PR #135767)
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Tue Apr 15 02:43:13 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Dominik Steenken (dominik-steenken)
<details>
<summary>Changes</summary>
When compiling with `-pg`, the `EntryExitInstrumenterPass` will insert calls to the glibc function `mcount` at the begining of each `MachineFunction`.
On SystemZ, these calls require special handling:
- The call to `mcount` needs to happen at the beginning of the prologue.
- Prior to the call to `mcount`, register `%r14`, the return address of the callee function, must be stored 8 bytes above the stack pointer `%r15`. After the call to `mcount` returns, that register needs to be restored.
This commit adds some special handling to the EntryExitInstrumenterPass that keeps the insertion of the mcount function into the module, but skips over insertion of the actual call in order to perform this insertion in the `emitPrologue` function. There, a simple sequence of store/call/load is inserted, which implements the above.
Noe that the desired change in the `EntryExitInstrumenterPass` necessitated a change to the signature of the `insertCall` function, since there now is a possibility that no call is inserted, and thus that needs to be communicated to the caller. We can't ignore that fact either, because we need `Changed` to be `false` in the caller for `SystemZ`, so we can use it in a guard to prevent the deletion of the `mcount` attribute from the function, which we still need for our postprocessing, aka call insertion, in `emitPrologue`.
This PR should address issue #<!-- -->121137 . The test inserted here is derived from the example given in that issue.
---
Full diff: https://github.com/llvm/llvm-project/pull/135767.diff
3 Files Affected:
- (modified) llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp (+28)
- (modified) llvm/lib/Transforms/Utils/EntryExitInstrumenter.cpp (+10-6)
- (added) llvm/test/CodeGen/SystemZ/mcount.ll (+35)
``````````diff
diff --git a/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp b/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
index 9561ea544b270..0936690dafdcd 100644
--- a/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
@@ -7,6 +7,7 @@
//===----------------------------------------------------------------------===//
#include "SystemZFrameLowering.h"
+#include "MCTargetDesc/SystemZMCTargetDesc.h"
#include "SystemZCallingConv.h"
#include "SystemZInstrInfo.h"
#include "SystemZMachineFunctionInfo.h"
@@ -18,6 +19,8 @@
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/CodeGen/TargetLoweringObjectFileImpl.h"
#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Module.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
@@ -558,6 +561,31 @@ void SystemZELFFrameLowering::emitPrologue(MachineFunction &MF,
// to determine the end of the prologue.
DebugLoc DL;
+ // Add mcount instrumentation if necessary.
+ if (MF.getFunction().getFnAttribute("instrument-function-entry-inlined").getValueAsString() == "mcount") {
+
+ // Store return address 8 bytes above stack pointer.
+ BuildMI(MBB, MBBI, DL, ZII->get(SystemZ::STG))
+ .addReg(SystemZ::R14D)
+ .addReg(SystemZ::R15D)
+ .addImm(8)
+ .addReg(0);
+
+ // Call mcount (Regmask 0 to ensure this will not be moved by the
+ // scheduler.).
+ const uint32_t Mask = 0;
+ BuildMI(MBB, MBBI, DL, ZII->get(SystemZ::CallBRASL))
+ .addGlobalAddress(MF.getFunction().getParent()->getFunction("mcount"))
+ .addRegMask(&Mask);
+
+ // Reload return address drom 8 bytes above stack pointer.
+ BuildMI(MBB, MBBI, DL, ZII->get(SystemZ::LG))
+ .addReg(SystemZ::R14D)
+ .addReg(SystemZ::R15D)
+ .addImm(8)
+ .addReg(0);
+ }
+
// The current offset of the stack pointer from the CFA.
int64_t SPOffsetFromCFA = -SystemZMC::ELFCFAOffsetFromInitialSP;
diff --git a/llvm/lib/Transforms/Utils/EntryExitInstrumenter.cpp b/llvm/lib/Transforms/Utils/EntryExitInstrumenter.cpp
index d47f1b4253b54..72e4f8791a91a 100644
--- a/llvm/lib/Transforms/Utils/EntryExitInstrumenter.cpp
+++ b/llvm/lib/Transforms/Utils/EntryExitInstrumenter.cpp
@@ -22,7 +22,7 @@
using namespace llvm;
-static void insertCall(Function &CurFn, StringRef Func,
+static bool insertCall(Function &CurFn, StringRef Func,
BasicBlock::iterator InsertionPt, DebugLoc DL) {
Module &M = *InsertionPt->getParent()->getParent()->getParent();
LLVMContext &C = InsertionPt->getParent()->getContext();
@@ -63,12 +63,16 @@ static void insertCall(Function &CurFn, StringRef Func,
false));
CallInst *Call = CallInst::Create(Fn, RetAddr, "", InsertionPt);
Call->setDebugLoc(DL);
+ } else if (TargetTriple.isSystemZ()) {
+ M.getOrInsertFunction(Func, Type::getVoidTy(C));
+ // skip insertion for `mcount` on SystemZ. This will be handled later in `emitPrologue`.
+ return false;
} else {
FunctionCallee Fn = M.getOrInsertFunction(Func, Type::getVoidTy(C));
CallInst *Call = CallInst::Create(Fn, "", InsertionPt);
Call->setDebugLoc(DL);
}
- return;
+ return true;
}
if (Func == "__cyg_profile_func_enter" || Func == "__cyg_profile_func_exit") {
@@ -87,7 +91,7 @@ static void insertCall(Function &CurFn, StringRef Func,
CallInst *Call =
CallInst::Create(Fn, ArrayRef<Value *>(Args), "", InsertionPt);
Call->setDebugLoc(DL);
- return;
+ return true;
}
// We only know how to call a fixed set of instrumentation functions, because
@@ -129,9 +133,9 @@ static bool runOnFunction(Function &F, bool PostInlining) {
if (auto SP = F.getSubprogram())
DL = DILocation::get(SP->getContext(), SP->getScopeLine(), 0, SP);
- insertCall(F, EntryFunc, F.begin()->getFirstInsertionPt(), DL);
- Changed = true;
- F.removeFnAttr(EntryAttr);
+ Changed = insertCall(F, EntryFunc, F.begin()->getFirstInsertionPt(), DL);
+ if (Changed)
+ F.removeFnAttr(EntryAttr);
}
if (!ExitFunc.empty()) {
diff --git a/llvm/test/CodeGen/SystemZ/mcount.ll b/llvm/test/CodeGen/SystemZ/mcount.ll
new file mode 100644
index 0000000000000..01bd34548f125
--- /dev/null
+++ b/llvm/test/CodeGen/SystemZ/mcount.ll
@@ -0,0 +1,35 @@
+; Test proper insertion of mcount instrumentation
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu -o - | FileCheck %s
+;
+; CHECK: # %bb.0:
+; CHECK-NEXT: stg %r14, 8(%r15)
+; CHECK-NEXT: brasl %r14, mcount at PLT
+; CHECK-NEXT: lg %r14, 8(%r15)
+define dso_local signext i32 @fib(i32 noundef signext %n) #0 {
+entry:
+ %n.addr = alloca i32, align 4
+ store i32 %n, ptr %n.addr, align 4
+ %0 = load i32, ptr %n.addr, align 4
+ %cmp = icmp sle i32 %0, 1
+ br i1 %cmp, label %cond.true, label %cond.false
+
+cond.true: ; preds = %entry
+ br label %cond.end
+
+cond.false: ; preds = %entry
+ %1 = load i32, ptr %n.addr, align 4
+ %sub = sub nsw i32 %1, 1
+ %call = call signext i32 @fib(i32 noundef signext %sub)
+ %2 = load i32, ptr %n.addr, align 4
+ %sub1 = sub nsw i32 %2, 2
+ %call2 = call signext i32 @fib(i32 noundef signext %sub1)
+ %add = add nsw i32 %call, %call2
+ br label %cond.end
+
+cond.end: ; preds = %cond.false, %cond.true
+ %cond = phi i32 [ 1, %cond.true ], [ %add, %cond.false ]
+ ret i32 %cond
+}
+
+attributes #0 = { "instrument-function-entry-inlined"="mcount" }
``````````
</details>
https://github.com/llvm/llvm-project/pull/135767
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