[lld] r360685 - [LLD][ELF] Add the -z ifunc-noplt option
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Tue May 14 08:25:21 PDT 2019
Author: maskray
Date: Tue May 14 08:25:21 2019
New Revision: 360685
URL: http://llvm.org/viewvc/llvm-project?rev=360685&view=rev
Log:
[LLD][ELF] Add the -z ifunc-noplt option
Patch by Mark Johnston!
Summary:
When the option is configured, ifunc calls do not go through the PLT;
rather, they appear as regular function calls with relocations
referencing the ifunc symbol, and the resolver is invoked when
applying the relocation. This is intended for use in freestanding
environments where text relocations are permissible and is incompatible
with the -z text option. The option is motivated by ifunc usage in the
FreeBSD kernel, where ifuncs are used to elide CPU feature flag bit
checks in hot paths. Instead of replacing the cost of a branch with that
of an indirect function call, the -z ifunc-noplt option is used to ensure
that ifunc calls carry no hidden overhead relative to normal function
calls.
Test Plan:
I added a couple of regression tests and tested the FreeBSD kernel
build using the latest lld sources.
To demonstrate the effects of the change, I used a micro-benchmark
which results in frequent invocations of a FreeBSD kernel ifunc. The
benchmark was run with and without IBRS enabled, and with and without
-zifunc-noplt configured. The observed speedup is small and consistent,
and is significantly larger with IBRS enabled:
https://people.freebsd.org/~markj/ifunc-noplt/noibrs.txt
https://people.freebsd.org/~markj/ifunc-noplt/ibrs.txt
Reviewed By: ruiu, MaskRay
Differential Revision: https://reviews.llvm.org/D61613
Added:
lld/trunk/test/ELF/gnu-ifunc-noplt-i386.s
lld/trunk/test/ELF/gnu-ifunc-noplt.s
Modified:
lld/trunk/ELF/Config.h
lld/trunk/ELF/Driver.cpp
lld/trunk/ELF/Relocations.cpp
lld/trunk/docs/ld.lld.1
lld/trunk/test/ELF/basic.s
Modified: lld/trunk/ELF/Config.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Config.h?rev=360685&r1=360684&r2=360685&view=diff
==============================================================================
--- lld/trunk/ELF/Config.h (original)
+++ lld/trunk/ELF/Config.h Tue May 14 08:25:21 2019
@@ -194,6 +194,7 @@ struct Configuration {
bool ZExecstack;
bool ZGlobal;
bool ZHazardplt;
+ bool ZIfuncNoplt;
bool ZInitfirst;
bool ZInterpose;
bool ZKeepTextSectionPrefix;
Modified: lld/trunk/ELF/Driver.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Driver.cpp?rev=360685&r1=360684&r2=360685&view=diff
==============================================================================
--- lld/trunk/ELF/Driver.cpp (original)
+++ lld/trunk/ELF/Driver.cpp Tue May 14 08:25:21 2019
@@ -309,6 +309,9 @@ static void checkOptions() {
if (!Config->Relocatable && !Config->DefineCommon)
error("-no-define-common not supported in non relocatable output");
+ if (Config->ZText && Config->ZIfuncNoplt)
+ error("-z text and -z ifunc-noplt may not be used together");
+
if (Config->Relocatable) {
if (Config->Shared)
error("-r and -shared may not be used together");
@@ -358,7 +361,7 @@ static bool getZFlag(opt::InputArgList &
static bool isKnownZFlag(StringRef S) {
return S == "combreloc" || S == "copyreloc" || S == "defs" ||
S == "execstack" || S == "global" || S == "hazardplt" ||
- S == "initfirst" || S == "interpose" ||
+ S == "ifunc-noplt" || S == "initfirst" || S == "interpose" ||
S == "keep-text-section-prefix" || S == "lazy" || S == "muldefs" ||
S == "nocombreloc" || S == "nocopyreloc" || S == "nodefaultlib" ||
S == "nodelete" || S == "nodlopen" || S == "noexecstack" ||
@@ -896,6 +899,7 @@ static void readConfigs(opt::InputArgLis
Config->ZExecstack = getZFlag(Args, "execstack", "noexecstack", false);
Config->ZGlobal = hasZOption(Args, "global");
Config->ZHazardplt = hasZOption(Args, "hazardplt");
+ Config->ZIfuncNoplt = hasZOption(Args, "ifunc-noplt");
Config->ZInitfirst = hasZOption(Args, "initfirst");
Config->ZInterpose = hasZOption(Args, "interpose");
Config->ZKeepTextSectionPrefix = getZFlag(
Modified: lld/trunk/ELF/Relocations.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.cpp?rev=360685&r1=360684&r2=360685&view=diff
==============================================================================
--- lld/trunk/ELF/Relocations.cpp (original)
+++ lld/trunk/ELF/Relocations.cpp Tue May 14 08:25:21 2019
@@ -1066,7 +1066,7 @@ static void scanReloc(InputSectionBase &
// be resolved within the executable will actually be resolved that way at
// runtime, because the main exectuable is always at the beginning of a search
// list. We can leverage that fact.
- if (!Sym.IsPreemptible && !Sym.isGnuIFunc()) {
+ if (!Sym.IsPreemptible && (!Sym.isGnuIFunc() || Config->ZIfuncNoplt)) {
if (Expr == R_GOT_PC && !isAbsoluteValue(Sym))
Expr = Target->adjustRelaxExpr(Type, RelocatedAddr, Expr);
else
@@ -1094,6 +1094,14 @@ static void scanReloc(InputSectionBase &
return;
}
+ // We were asked not to generate PLT entries for ifuncs. Instead, pass the
+ // direct relocation on through.
+ if (Sym.isGnuIFunc() && Config->ZIfuncNoplt) {
+ Sym.ExportDynamic = true;
+ In.RelaDyn->addReloc(Type, &Sec, Offset, &Sym, Addend, R_ADDEND, Type);
+ return;
+ }
+
// Non-preemptible ifuncs require special handling. First, handle the usual
// case where the symbol isn't one of these.
if (!Sym.isGnuIFunc() || Sym.IsPreemptible) {
Modified: lld/trunk/docs/ld.lld.1
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/docs/ld.lld.1?rev=360685&r1=360684&r2=360685&view=diff
==============================================================================
--- lld/trunk/docs/ld.lld.1 (original)
+++ lld/trunk/docs/ld.lld.1 Tue May 14 08:25:21 2019
@@ -4,7 +4,7 @@
.\"
.\" This man page documents only lld's ELF linking support, obtained originally
.\" from FreeBSD.
-.Dd February 26, 2019
+.Dd May 12, 2019
.Dt LD.LLD 1
.Os
.Sh NAME
@@ -552,6 +552,14 @@ Sets the
.Dv DYNAMIC
section.
Different loaders can decide how to handle this flag on their own.
+.It Cm ifunc-noplt
+Do not emit PLT entries for ifunc symbols.
+Instead, emit text relocations referencing the resolver.
+This is an experimental optimization and only suitable for standalone
+environments where text relocations do not have the usual drawbacks.
+This option must be combined with the
+.Fl z Li notext
+option.
.It Cm initfirst
Sets the
.Dv DF_1_INITFIRST
Modified: lld/trunk/test/ELF/basic.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/basic.s?rev=360685&r1=360684&r2=360685&view=diff
==============================================================================
--- lld/trunk/test/ELF/basic.s (original)
+++ lld/trunk/test/ELF/basic.s Tue May 14 08:25:21 2019
@@ -252,3 +252,6 @@ _start:
# RUN: not ld.lld %t --thinlto-jobs=0 2>&1 | FileCheck --check-prefix=NOTHREADSTHIN %s
# RUN: not ld.lld %t --plugin-opt=jobs=0 2>&1 | FileCheck --check-prefix=NOTHREADSTHIN %s
# NOTHREADSTHIN: --thinlto-jobs: number of threads must be > 0
+
+# RUN: not ld.lld %t -z ifunc-noplt -z text 2>&1 | FileCheck --check-prefix=NOIFUNCPLTNOTEXTREL %s
+# NOIFUNCPLTNOTEXTREL: -z text and -z ifunc-noplt may not be used together
Added: lld/trunk/test/ELF/gnu-ifunc-noplt-i386.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/gnu-ifunc-noplt-i386.s?rev=360685&view=auto
==============================================================================
--- lld/trunk/test/ELF/gnu-ifunc-noplt-i386.s (added)
+++ lld/trunk/test/ELF/gnu-ifunc-noplt-i386.s Tue May 14 08:25:21 2019
@@ -0,0 +1,71 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=i686-pc-freebsd %S/Inputs/shared2-x86-64.s -o %t1.o
+// RUN: ld.lld %t1.o --shared -o %t.so
+// RUN: llvm-mc -filetype=obj -triple=i686-pc-freebsd %s -o %t.o
+// RUN: ld.lld -z ifunc-noplt -z notext --hash-style=sysv %t.so %t.o -o %tout
+// RUN: llvm-objdump -d --no-show-raw-insn %tout | FileCheck %s --check-prefix=DISASM
+// RUN: llvm-readobj -r --dynamic-table %tout | FileCheck %s
+
+// Check that we emitted relocations for the ifunc calls
+// CHECK: Relocations [
+// CHECK-NEXT: Section (4) .rel.dyn {
+// CHECK-NEXT: 0x401008 R_386_PLT32 bar
+// CHECK-NEXT: 0x401003 R_386_PLT32 foo
+// CHECK-NEXT: }
+// CHECK-NEXT: Section (5) .rel.plt {
+// CHECK-NEXT: 0x40300C R_386_JUMP_SLOT bar2
+// CHECK-NEXT: 0x403010 R_386_JUMP_SLOT zed2
+// CHECK-NEXT: }
+
+// Check that ifunc call sites still require relocation
+// DISASM: Disassembly of section .text:
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000401000 foo:
+// DISASM-NEXT: 401000: retl
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000401001 bar:
+// DISASM-NEXT: 401001: retl
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000401002 _start:
+// DISASM-NEXT: 401002: calll -4 <_start+0x1>
+// DISASM-NEXT: 401007: calll -4 <_start+0x6>
+// DISASM-NEXT: 40100c: calll 31 <bar2 at plt>
+// DISASM-NEXT: 401011: calll 42 <zed2 at plt>
+// DISASM-EMPTY:
+// DISASM-NEXT: Disassembly of section .plt:
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000401020 .plt:
+// DISASM-NEXT: 401020: pushl 4206596
+// DISASM-NEXT: 401026: jmpl *4206600
+// DISASM-NEXT: 40102c: nop
+// DISASM-NEXT: 40102d: nop
+// DISASM-NEXT: 40102e: nop
+// DISASM-NEXT: 40102f: nop
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000401030 bar2 at plt:
+// DISASM-NEXT: 401030: jmpl *4206604
+// DISASM-NEXT: 401036: pushl $0
+// DISASM-NEXT: 40103b: jmp -32 <.plt>
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000401040 zed2 at plt:
+// DISASM-NEXT: 401040: jmpl *4206608
+// DISASM-NEXT: 401046: pushl $8
+// DISASM-NEXT: 40104b: jmp -48 <.plt>
+
+.text
+.type foo STT_GNU_IFUNC
+.globl foo
+foo:
+ ret
+
+.type bar STT_GNU_IFUNC
+.globl bar
+bar:
+ ret
+
+.globl _start
+_start:
+ call foo at plt
+ call bar at plt
+ call bar2 at plt
+ call zed2 at plt
Added: lld/trunk/test/ELF/gnu-ifunc-noplt.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/gnu-ifunc-noplt.s?rev=360685&view=auto
==============================================================================
--- lld/trunk/test/ELF/gnu-ifunc-noplt.s (added)
+++ lld/trunk/test/ELF/gnu-ifunc-noplt.s Tue May 14 08:25:21 2019
@@ -0,0 +1,68 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-freebsd %S/Inputs/shared2-x86-64.s -o %t1.o
+// RUN: ld.lld %t1.o --shared -o %t.so
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-freebsd %s -o %t.o
+// RUN: ld.lld -z ifunc-noplt -z notext --hash-style=sysv %t.so %t.o -o %tout
+// RUN: llvm-objdump -d --no-show-raw-insn %tout | FileCheck %s --check-prefix=DISASM
+// RUN: llvm-readobj -r --dynamic-table %tout | FileCheck %s
+
+// Check that we emitted relocations for the ifunc calls
+// CHECK: Relocations [
+// CHECK-NEXT: Section (4) .rela.dyn {
+// CHECK-NEXT: 0x201008 R_X86_64_PLT32 bar 0xFFFFFFFFFFFFFFFC
+// CHECK-NEXT: 0x201003 R_X86_64_PLT32 foo 0xFFFFFFFFFFFFFFFC
+// CHECK-NEXT: }
+// CHECK-NEXT: Section (5) .rela.plt {
+// CHECK-NEXT: 0x203018 R_X86_64_JUMP_SLOT bar2 0x0
+// CHECK-NEXT: 0x203020 R_X86_64_JUMP_SLOT zed2 0x0
+// CHECK-NEXT: }
+
+// Check that ifunc call sites still require relocation
+// DISASM: Disassembly of section .text:
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000201000 foo:
+// DISASM-NEXT: 201000: retq
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000201001 bar:
+// DISASM-NEXT: 201001: retq
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000201002 _start:
+// DISASM-NEXT: 201002: callq 0 <_start+0x5>
+// DISASM-NEXT: 201007: callq 0 <_start+0xa>
+// DISASM-NEXT: 20100c: callq 31 <bar2 at plt>
+// DISASM-NEXT: 201011: callq 42 <zed2 at plt>
+// DISASM-EMPTY:
+// DISASM-NEXT: Disassembly of section .plt:
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000201020 .plt:
+// DISASM-NEXT: 201020: pushq 8162(%rip)
+// DISASM-NEXT: 201026: jmpq *8164(%rip)
+// DISASM-NEXT: 20102c: nopl (%rax)
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000201030 bar2 at plt:
+// DISASM-NEXT: 201030: jmpq *8162(%rip)
+// DISASM-NEXT: 201036: pushq $0
+// DISASM-NEXT: 20103b: jmp -32 <.plt>
+// DISASM-EMPTY:
+// DISASM-NEXT: 0000000000201040 zed2 at plt:
+// DISASM-NEXT: 201040: jmpq *8154(%rip)
+// DISASM-NEXT: 201046: pushq $1
+// DISASM-NEXT: 20104b: jmp -48 <.plt>
+
+.text
+.type foo STT_GNU_IFUNC
+.globl foo
+foo:
+ ret
+
+.type bar STT_GNU_IFUNC
+.globl bar
+bar:
+ ret
+
+.globl _start
+_start:
+ call foo
+ call bar
+ call bar2
+ call zed2
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