[polly] f992ae4 - Slightly improve the getenv("bar") linking problem
Luke Drummond via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 23 04:15:24 PDT 2025
Author: Luke Drummond
Date: 2025-07-23T12:14:51+01:00
New Revision: f992ae4fd16357116b341a1c8291b970787dc462
URL: https://github.com/llvm/llvm-project/commit/f992ae4fd16357116b341a1c8291b970787dc462
DIFF: https://github.com/llvm/llvm-project/commit/f992ae4fd16357116b341a1c8291b970787dc462.diff
LOG: Slightly improve the getenv("bar") linking problem
There's been a variation of the following in the code since 2005:
if (unoptimizable_true)
return;
use_this_symbol_to_force_linking(); // unreachable but never removed
Way back in 00d5508496c it was the win32 call `GetCurrentProcess`
but switched to `getenv("bar")` fairly soon after in 63e504ff43. While
that pulled in fewer dependencies and made the code portable, it's a
bit of a weird construct. The environment variable used for the `getenv`
call is "bar", which is particularly weird to see fly past when you run
`ltrace` on a binary linked against LLVM.
In this patch I don't try to replace this construct wholesale - it's
still required for architectural reasons I'm not able to tackle right
now, but I did try and make it slightly less weird and opaque:
- It gives the construct a name
- The environment variable hints where this comes from and that its
value is ignored
Combined, this should be a bit of improvement for the next person who
wonders what LLVM is up to when they trace their process or see
smatterings of `getenv("bar")` dotted around the source.
Added:
llvm/include/llvm/Support/AlwaysTrue.h
Modified:
llvm/include/llvm/CodeGen/LinkAllAsmWriterComponents.h
llvm/include/llvm/CodeGen/LinkAllCodegenComponents.h
llvm/include/llvm/ExecutionEngine/MCJIT.h
llvm/include/llvm/LinkAllIR.h
llvm/include/llvm/LinkAllPasses.h
llvm/tools/bugpoint/bugpoint.cpp
polly/include/polly/LinkAllPasses.h
Removed:
################################################################################
diff --git a/llvm/include/llvm/CodeGen/LinkAllAsmWriterComponents.h b/llvm/include/llvm/CodeGen/LinkAllAsmWriterComponents.h
index c22f9d49f374b..c70413d72ebfd 100644
--- a/llvm/include/llvm/CodeGen/LinkAllAsmWriterComponents.h
+++ b/llvm/include/llvm/CodeGen/LinkAllAsmWriterComponents.h
@@ -15,19 +15,17 @@
#define LLVM_CODEGEN_LINKALLASMWRITERCOMPONENTS_H
#include "llvm/IR/BuiltinGCs.h"
-#include <cstdlib>
+#include "llvm/Support/AlwaysTrue.h"
namespace {
struct ForceAsmWriterLinking {
ForceAsmWriterLinking() {
// We must reference the plug-ins in such a way that compilers will not
// delete it all as dead code, even with whole program optimization,
- // yet is effectively a NO-OP. As the compiler isn't smart enough
- // to know that getenv() never returns -1, this will do the job.
- // This is so that globals in the translation units where these functions
- // are defined are forced to be initialized, populating various
- // registries.
- if (std::getenv("bar") != (char*) -1)
+ // yet is effectively a NO-OP. This is so that globals in the translation
+ // units where these functions are defined are forced to be initialized,
+ // populating various registries.
+ if (llvm::getNonFoldableAlwaysTrue())
return;
llvm::linkOcamlGCPrinter();
diff --git a/llvm/include/llvm/CodeGen/LinkAllCodegenComponents.h b/llvm/include/llvm/CodeGen/LinkAllCodegenComponents.h
index 6f56682dce5fa..f0a01d2baf0e0 100644
--- a/llvm/include/llvm/CodeGen/LinkAllCodegenComponents.h
+++ b/llvm/include/llvm/CodeGen/LinkAllCodegenComponents.h
@@ -16,20 +16,18 @@
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/SchedulerRegistry.h"
+#include "llvm/Support/AlwaysTrue.h"
#include "llvm/Target/TargetMachine.h"
-#include <cstdlib>
namespace {
struct ForceCodegenLinking {
ForceCodegenLinking() {
// We must reference the passes in such a way that compilers will not
// delete it all as dead code, even with whole program optimization,
- // yet is effectively a NO-OP. As the compiler isn't smart enough
- // to know that getenv() never returns -1, this will do the job.
- // This is so that globals in the translation units where these functions
- // are defined are forced to be initialized, populating various
- // registries.
- if (std::getenv("bar") != (char*) -1)
+ // yet is effectively a NO-OP. This is so that globals in the translation
+ // units where these functions are defined are forced to be initialized,
+ // populating various registries.
+ if (llvm::getNonFoldableAlwaysTrue())
return;
(void) llvm::createFastRegisterAllocator();
diff --git a/llvm/include/llvm/ExecutionEngine/MCJIT.h b/llvm/include/llvm/ExecutionEngine/MCJIT.h
index c836c06813fc6..1e035c0008fbd 100644
--- a/llvm/include/llvm/ExecutionEngine/MCJIT.h
+++ b/llvm/include/llvm/ExecutionEngine/MCJIT.h
@@ -15,8 +15,8 @@
#define LLVM_EXECUTIONENGINE_MCJIT_H
#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/Support/AlwaysTrue.h"
#include "llvm/Support/Compiler.h"
-#include <cstdlib>
extern "C" LLVM_ABI void LLVMLinkInMCJIT();
@@ -24,13 +24,11 @@ namespace {
struct ForceMCJITLinking {
ForceMCJITLinking() {
// We must reference MCJIT in such a way that compilers will not
- // delete it all as dead code, even with whole program optimization,
- // yet is effectively a NO-OP. As the compiler isn't smart enough
- // to know that getenv() never returns -1, this will do the job.
- // This is so that globals in the translation units where these functions
- // are defined are forced to be initialized, populating various
- // registries.
- if (std::getenv("bar") != (char*) -1)
+ // delete it all as dead code, even with whole program optimization, yet
+ // is effectively a NO-OP. This is so that globals in the translation
+ // units where these functions are defined are forced to be initialized,
+ // populating various registries.
+ if (llvm::getNonFoldableAlwaysTrue())
return;
LLVMLinkInMCJIT();
diff --git a/llvm/include/llvm/LinkAllIR.h b/llvm/include/llvm/LinkAllIR.h
index ceed784d557de..894a8dd5bd926 100644
--- a/llvm/include/llvm/LinkAllIR.h
+++ b/llvm/include/llvm/LinkAllIR.h
@@ -21,6 +21,7 @@
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Verifier.h"
+#include "llvm/Support/AlwaysTrue.h"
#include "llvm/Support/DynamicLibrary.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/Memory.h"
@@ -29,19 +30,16 @@
#include "llvm/Support/Process.h"
#include "llvm/Support/Program.h"
#include "llvm/Support/Signals.h"
-#include <cstdlib>
namespace {
struct ForceVMCoreLinking {
ForceVMCoreLinking() {
// We must reference VMCore in such a way that compilers will not
- // delete it all as dead code, even with whole program optimization,
- // yet is effectively a NO-OP. As the compiler isn't smart enough
- // to know that getenv() never returns -1, this will do the job.
+ // delete it all as dead code, even with whole program optimization.
// This is so that globals in the translation units where these functions
// are defined are forced to be initialized, populating various
// registries.
- if (std::getenv("bar") != (char*) -1)
+ if (llvm::getNonFoldableAlwaysTrue())
return;
llvm::LLVMContext Context;
(void)new llvm::Module("", Context);
diff --git a/llvm/include/llvm/LinkAllPasses.h b/llvm/include/llvm/LinkAllPasses.h
index bae7f0da7022c..f82a43967e67a 100644
--- a/llvm/include/llvm/LinkAllPasses.h
+++ b/llvm/include/llvm/LinkAllPasses.h
@@ -34,6 +34,7 @@
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRPrintingPasses.h"
+#include "llvm/Support/AlwaysTrue.h"
#include "llvm/Support/Valgrind.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/IPO/AlwaysInliner.h"
@@ -54,14 +55,12 @@ class Triple;
namespace {
struct ForcePassLinking {
ForcePassLinking() {
- // We must reference the passes in such a way that compilers will not
- // delete it all as dead code, even with whole program optimization,
- // yet is effectively a NO-OP. As the compiler isn't smart enough
- // to know that getenv() never returns -1, this will do the job.
- // This is so that globals in the translation units where these functions
- // are defined are forced to be initialized, populating various
- // registries.
- if (std::getenv("bar") != (char *)-1)
+ // We must reference the passes in such a way that compilers will not delete
+ // it all as dead code, even with whole program optimization, yet is
+ // effectively a NO-OP. This is so that globals in the translation units
+ // where these functions are defined are forced to be initialized,
+ // populating various registries.
+ if (llvm::getNonFoldableAlwaysTrue())
return;
(void)llvm::createAtomicExpandLegacyPass();
diff --git a/llvm/include/llvm/Support/AlwaysTrue.h b/llvm/include/llvm/Support/AlwaysTrue.h
new file mode 100644
index 0000000000000..b6968568c11a0
--- /dev/null
+++ b/llvm/include/llvm/Support/AlwaysTrue.h
@@ -0,0 +1,25 @@
+//===--- AlwaysTrue.h - Helper for oqaque truthy values --*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_ALWAYS_TRUE_H
+#define LLVM_SUPPORT_ALWAYS_TRUE_H
+
+#include <cstdlib>
+
+namespace llvm {
+inline bool getNonFoldableAlwaysTrue() {
+ // Some parts of the codebase require a "constant true value" used as a
+ // predicate. These cases require that even with LTO and static linking,
+ // it's not possible for the compiler to fold the value. As compilers
+ // aren't smart enough to know that getenv() never returns -1, this will do
+ // the job.
+ return std::getenv("LLVM_IGNORED_ENV_VAR") != (char *)-1;
+}
+} // end namespace llvm
+
+#endif // LLVM_SUPPORT_ALWAYS_TRUE_H
diff --git a/llvm/tools/bugpoint/bugpoint.cpp b/llvm/tools/bugpoint/bugpoint.cpp
index e49efdfe7c8e0..87581e80a2496 100644
--- a/llvm/tools/bugpoint/bugpoint.cpp
+++ b/llvm/tools/bugpoint/bugpoint.cpp
@@ -22,6 +22,7 @@
#include "llvm/LinkAllIR.h"
#include "llvm/LinkAllPasses.h"
#include "llvm/Passes/PassPlugin.h"
+#include "llvm/Support/AlwaysTrue.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/InitLLVM.h"
#include "llvm/Support/PluginLoader.h"
@@ -111,7 +112,7 @@ int main(int argc, char **argv) {
initializeInstCombine(Registry);
initializeTarget(Registry);
- if (std::getenv("bar") == (char*) -1) {
+ if (!llvm::getNonFoldableAlwaysTrue()) {
InitializeAllTargets();
InitializeAllTargetMCs();
InitializeAllAsmPrinters();
diff --git a/polly/include/polly/LinkAllPasses.h b/polly/include/polly/LinkAllPasses.h
index a2f8f33299918..d2e10d1a7acc6 100644
--- a/polly/include/polly/LinkAllPasses.h
+++ b/polly/include/polly/LinkAllPasses.h
@@ -18,7 +18,7 @@
#include "polly/Support/DumpFunctionPass.h"
#include "polly/Support/DumpModulePass.h"
#include "llvm/ADT/StringRef.h"
-#include <cstdlib>
+#include "llvm/Support/AlwaysTrue.h"
namespace llvm {
class Pass;
@@ -72,11 +72,10 @@ extern char &CodePreparationID;
namespace {
struct PollyForcePassLinking {
PollyForcePassLinking() {
- // We must reference the passes in such a way that compilers will not
- // delete it all as dead code, even with whole program optimization,
- // yet is effectively a NO-OP. As the compiler isn't smart enough
- // to know that getenv() never returns -1, this will do the job.
- if (std::getenv("bar") != (char *)-1)
+ // We must reference the passes in such a way that compilers will not delete
+ // it all as dead code, even with whole program optimization, yet is
+ // effectively a NO-OP.
+ if (llvm::getNonFoldableAlwaysTrue())
return;
polly::createCodePreparationPass();
More information about the llvm-commits
mailing list