[clang] [llvm] Support -mnop-mcount and -mrecord-mcount on x86_64 elf targets. (PR #184641)
Paul Murphy via cfe-commits
cfe-commits at lists.llvm.org
Wed Mar 4 08:38:15 PST 2026
https://github.com/pmur created https://github.com/llvm/llvm-project/pull/184641
I found out these attributes are only support on s390x when prototyping my Rust [rfc](https://github.com/rust-lang/rfcs/pull/3917) for expanded instrumented function support.
Also, is this the correct/preferred way to insert nop's on x86?
>From 1d7b9d74e864672e507975983881e14a1c067f9f Mon Sep 17 00:00:00 2001
From: Paul Murphy <murp at redhat.com>
Date: Tue, 3 Mar 2026 14:43:18 -0600
Subject: [PATCH 1/2] [LLVM][CodeGen][X86] Support mnop-mcount and
mrecord-mcount attributes
Support mnop-mcount and mrecord-mcount attributes when used with the
fentry-call=true attribute.
Similar to SystemZ, use the mnop-mcount attribute to place a nop
instead of a call to __fentry__.
Likewise, use the mrecord-mcount attribute to record the location
of the call to fentry (or nop if used with the above) to a section
named __mcount_loc.
---
llvm/lib/Target/X86/X86ISelDAGToDAG.cpp | 14 ++++++++--
llvm/lib/Target/X86/X86MCInstLower.cpp | 18 ++++++++++++
llvm/test/CodeGen/X86/mnop-mcount-01.ll | 26 ++++++++++++++++++
llvm/test/CodeGen/X86/mnop-mcount-02.ll | 10 +++++++
llvm/test/CodeGen/X86/mrecord-mcount-01.ll | 32 ++++++++++++++++++++++
llvm/test/CodeGen/X86/mrecord-mcount-02.ll | 10 +++++++
6 files changed, 107 insertions(+), 3 deletions(-)
create mode 100644 llvm/test/CodeGen/X86/mnop-mcount-01.ll
create mode 100644 llvm/test/CodeGen/X86/mnop-mcount-02.ll
create mode 100644 llvm/test/CodeGen/X86/mrecord-mcount-01.ll
create mode 100644 llvm/test/CodeGen/X86/mrecord-mcount-02.ll
diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index 101ea3e231a5c..aa7efb8a05e1c 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -179,11 +179,19 @@ namespace {
bool runOnMachineFunction(MachineFunction &MF) override {
// Reset the subtarget each time through.
Subtarget = &MF.getSubtarget<X86Subtarget>();
- IndirectTlsSegRefs = MF.getFunction().hasFnAttribute(
- "indirect-tls-seg-refs");
+ const Function &F = MF.getFunction();
+
+ IndirectTlsSegRefs = F.hasFnAttribute("indirect-tls-seg-refs");
+
+ if (F.getFnAttribute("fentry-call").getValueAsString() != "true") {
+ if (F.hasFnAttribute("mnop-mcount"))
+ report_fatal_error("mnop-mcount only supported with fentry-call");
+ if (F.hasFnAttribute("mrecord-mcount"))
+ report_fatal_error("mrecord-mcount only supported with fentry-call");
+ }
// OptFor[Min]Size are used in pattern predicates that isel is matching.
- OptForMinSize = MF.getFunction().hasMinSize();
+ OptForMinSize = F.hasMinSize();
return SelectionDAGISel::runOnMachineFunction(MF);
}
diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index 4ef4718e1b01c..8bbd40d973fab 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -26,6 +26,7 @@
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringExtras.h"
+#include "llvm/BinaryFormat/ELF.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineFunction.h"
@@ -44,6 +45,7 @@
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCInstBuilder.h"
#include "llvm/MC/MCSection.h"
+#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/TargetRegistry.h"
@@ -885,6 +887,22 @@ void X86AsmPrinter::LowerFENTRY_CALL(const MachineInstr &MI,
X86MCInstLower &MCIL) {
bool Is64Bits = Subtarget->is64Bit();
MCContext &Ctx = OutStreamer->getContext();
+
+ if (MF->getFunction().hasFnAttribute("mrecord-mcount")) {
+ MCSymbol *DotSym = OutContext.createTempSymbol();
+ OutStreamer->pushSection();
+ OutStreamer->switchSection(
+ Ctx.getELFSection("__mcount_loc", ELF::SHT_PROGBITS, ELF::SHF_ALLOC));
+ OutStreamer->emitSymbolValue(DotSym, Is64Bits ? 8 : 4);
+ OutStreamer->popSection();
+ OutStreamer->emitLabel(DotSym);
+ }
+
+ if (MF->getFunction().hasFnAttribute("mnop-mcount")) {
+ emitX86Nops(*OutStreamer, 5, Subtarget);
+ return;
+ }
+
MCSymbol *fentry = Ctx.getOrCreateSymbol("__fentry__");
const MCSymbolRefExpr *Op = MCSymbolRefExpr::create(fentry, Ctx);
diff --git a/llvm/test/CodeGen/X86/mnop-mcount-01.ll b/llvm/test/CodeGen/X86/mnop-mcount-01.ll
new file mode 100644
index 0000000000000..ddefa497659f3
--- /dev/null
+++ b/llvm/test/CodeGen/X86/mnop-mcount-01.ll
@@ -0,0 +1,26 @@
+; RUN: llc %s -mtriple=x86_64-linux-gnu -o - -verify-machineinstrs \
+; RUN: | FileCheck %s
+
+define void @test1() #0 {
+entry:
+ ret void
+
+; CHECK-LABEL: @test1
+; CHECK: callq __fentry__
+; CHECK-NOT: nopl 8(%rax,%rax)
+; CHECK: retq
+}
+
+define void @test2() #1 {
+entry:
+ ret void
+
+; CHECK-LABEL: @test2
+; CHECK-NOT: callq __fentry__
+; CHECK: nopl 8(%rax,%rax)
+; CHECK: retq
+}
+
+attributes #0 = { "fentry-call"="true" }
+attributes #1 = { "fentry-call"="true" "mnop-mcount" }
+
diff --git a/llvm/test/CodeGen/X86/mnop-mcount-02.ll b/llvm/test/CodeGen/X86/mnop-mcount-02.ll
new file mode 100644
index 0000000000000..d576e0f289818
--- /dev/null
+++ b/llvm/test/CodeGen/X86/mnop-mcount-02.ll
@@ -0,0 +1,10 @@
+; RUN: not --crash llc %s -mtriple=x86_64-linux-gnu -o - 2>&1 | FileCheck %s
+;
+; CHECK: LLVM ERROR: mnop-mcount only supported with fentry-call
+
+define void @test1() #0 {
+entry:
+ ret void
+}
+
+attributes #0 = { "instrument-function-entry-inlined"="mcount" "mnop-mcount" }
diff --git a/llvm/test/CodeGen/X86/mrecord-mcount-01.ll b/llvm/test/CodeGen/X86/mrecord-mcount-01.ll
new file mode 100644
index 0000000000000..6202a652b8831
--- /dev/null
+++ b/llvm/test/CodeGen/X86/mrecord-mcount-01.ll
@@ -0,0 +1,32 @@
+; RUN: llc %s -mtriple=x86_64-linux-gnu -o - -verify-machineinstrs \
+; RUN: | FileCheck %s
+
+define void @test1() #0 {
+entry:
+ ret void
+
+; CHECK-LABEL: test1:
+; CHECK: .section __mcount_loc,"a", at progbits
+; CHECK: .quad .Ltmp0
+; CHECK: .text
+; CHECK: .Ltmp0:
+; CHECK: callq __fentry__
+; CHECK: retq
+}
+
+define void @test2() #1 {
+entry:
+ ret void
+
+; CHECK-LABEL: test2:
+; CHECK: .section __mcount_loc,"a", at progbits
+; CHECK: .quad .Ltmp1
+; CHECK: .text
+; CHECK: .Ltmp1:
+; CHECK: nopl 8(%rax,%rax)
+; CHECK-NOT: callq __fentry__
+; CHECK: retq
+}
+
+attributes #0 = { "fentry-call"="true" "mrecord-mcount" }
+attributes #1 = { "fentry-call"="true" "mnop-mcount" "mrecord-mcount" }
diff --git a/llvm/test/CodeGen/X86/mrecord-mcount-02.ll b/llvm/test/CodeGen/X86/mrecord-mcount-02.ll
new file mode 100644
index 0000000000000..a1f80f1840c41
--- /dev/null
+++ b/llvm/test/CodeGen/X86/mrecord-mcount-02.ll
@@ -0,0 +1,10 @@
+; RUN: not --crash llc %s -mtriple=x86_64-linux-gnu -o - 2>&1 | FileCheck %s
+;
+; CHECK: LLVM ERROR: mrecord-mcount only supported with fentry-call
+
+define void @test1() #0 {
+entry:
+ ret void
+}
+
+attributes #0 = { "instrument-function-entry-inlined"="mcount" "mrecord-mcount" }
>From 0c14b0aef3af7f3fe282a94513c7cc4cb9c10802 Mon Sep 17 00:00:00 2001
From: Paul Murphy <murp at redhat.com>
Date: Wed, 4 Mar 2026 08:47:16 -0600
Subject: [PATCH 2/2] [clang] enable -mnop-mcount -mrecord-mcount for x86_64
These are gated by -mfentry.
---
clang/lib/Driver/ToolChains/Clang.cpp | 6 ++++--
clang/test/CodeGen/mnop-mcount.c | 8 ++++++++
clang/test/CodeGen/mrecord-mcount.c | 8 ++++++++
clang/test/Driver/mcount.c | 4 +---
4 files changed, 21 insertions(+), 5 deletions(-)
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index e860136aae5b3..2807be2c7edd0 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6884,14 +6884,16 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
<< A->getAsString(Args) << TripleStr;
}
if (Arg *A = Args.getLastArg(options::OPT_mnop_mcount)) {
- if (Arch == llvm::Triple::systemz)
+ if (Arch == llvm::Triple::systemz || (TC.getTriple().isX86_64() &&
+ TC.getTriple().isOSBinFormatELF()))
A->render(Args, CmdArgs);
else
D.Diag(diag::err_drv_unsupported_opt_for_target)
<< A->getAsString(Args) << TripleStr;
}
if (Arg *A = Args.getLastArg(options::OPT_mrecord_mcount)) {
- if (Arch == llvm::Triple::systemz)
+ if (Arch == llvm::Triple::systemz || (TC.getTriple().isX86_64() &&
+ TC.getTriple().isOSBinFormatELF()))
A->render(Args, CmdArgs);
else
D.Diag(diag::err_drv_unsupported_opt_for_target)
diff --git a/clang/test/CodeGen/mnop-mcount.c b/clang/test/CodeGen/mnop-mcount.c
index ebb21cc560e37..31b4e995cff02 100644
--- a/clang/test/CodeGen/mnop-mcount.c
+++ b/clang/test/CodeGen/mnop-mcount.c
@@ -6,6 +6,14 @@
// RUN: %s 2>&1 | FileCheck -check-prefix=NOPG %s
// RUN: %clang_cc1 -mnop-mcount -triple s390x-ibm-linux -emit-llvm -o - %s \
// RUN: 2>&1 | FileCheck -check-prefix=NOPG %s
+// RUN: %clang_cc1 -pg -mfentry -mnop-mcount -triple x86_64-ibm-linux -emit-llvm \
+// RUN: -o - %s 2>&1 | FileCheck %s
+// RUN: not %clang_cc1 -pg -mnop-mcount -triple x86_64-ibm-linux -emit-llvm -o - \
+// RUN: %s 2>&1 | FileCheck -check-prefix=NOMFENTRY %s
+// RUN: %clang_cc1 -mfentry -mnop-mcount -triple x86_64-ibm-linux -emit-llvm -o - \
+// RUN: %s 2>&1 | FileCheck -check-prefix=NOPG %s
+// RUN: %clang_cc1 -mnop-mcount -triple x86_64-ibm-linux -emit-llvm -o - %s \
+// RUN: 2>&1 | FileCheck -check-prefix=NOPG %s
int foo(void) {
return 0;
diff --git a/clang/test/CodeGen/mrecord-mcount.c b/clang/test/CodeGen/mrecord-mcount.c
index 292a807073cef..832d4ee5e3e9f 100644
--- a/clang/test/CodeGen/mrecord-mcount.c
+++ b/clang/test/CodeGen/mrecord-mcount.c
@@ -6,6 +6,14 @@
// RUN: %s 2>&1 | FileCheck -check-prefix=NOPG %s
// RUN: %clang_cc1 -mrecord-mcount -triple s390x-ibm-linux -emit-llvm -o - %s \
// RUN: 2>&1 | FileCheck -check-prefix=NOPG %s
+// RUN: %clang_cc1 -pg -mfentry -mrecord-mcount -triple x86_64-ibm-linux -emit-llvm \
+// RUN: -o - %s 2>&1 | FileCheck %s
+// RUN: not %clang_cc1 -pg -mrecord-mcount -triple x86_64-ibm-linux -emit-llvm -o - \
+// RUN: %s 2>&1 | FileCheck -check-prefix=NOMFENTRY %s
+// RUN: %clang_cc1 -mfentry -mrecord-mcount -triple x86_64-ibm-linux -emit-llvm -o - \
+// RUN: %s 2>&1 | FileCheck -check-prefix=NOPG %s
+// RUN: %clang_cc1 -mrecord-mcount -triple x86_64-ibm-linux -emit-llvm -o - %s \
+// RUN: 2>&1 | FileCheck -check-prefix=NOPG %s
int foo(void) {
return 0;
diff --git a/clang/test/Driver/mcount.c b/clang/test/Driver/mcount.c
index 70f554745b9c3..72d48b3806fe7 100644
--- a/clang/test/Driver/mcount.c
+++ b/clang/test/Driver/mcount.c
@@ -1,12 +1,10 @@
// RUN: %clang --target=s390x -c -### %s -mnop-mcount -mrecord-mcount 2>&1 | FileCheck %s
+// RUN: %clang --target=x86_64 -c -### %s -mnop-mcount -mrecord-mcount 2>&1 | FileCheck %s
// CHECK: "-mnop-mcount"
// CHECK: "-mrecord-mcount"
-// RUN: not %clang --target=x86_64 -c -### %s -mnop-mcount -mrecord-mcount 2>&1 | FileCheck --check-prefix=ERR1 %s
// RUN: not %clang --target=aarch64 -c -### %s -mnop-mcount -mrecord-mcount 2>&1 | FileCheck --check-prefix=ERR2 %s
-// ERR1: error: unsupported option '-mnop-mcount' for target 'x86_64'
-// ERR1: error: unsupported option '-mrecord-mcount' for target 'x86_64'
// ERR2: error: unsupported option '-mnop-mcount' for target 'aarch64'
// ERR2: error: unsupported option '-mrecord-mcount' for target 'aarch64'
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