[llvm] 2cb8d5c - [Pseudo Probe] Do not place functions in nodeduplicate COMDATs

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2023-06-17T15:40:20-07:00
New Revision: 2cb8d5ca3ad01c71b6481bd09779b4e82ccf8196

URL: https://github.com/llvm/llvm-project/commit/2cb8d5ca3ad01c71b6481bd09779b4e82ccf8196
DIFF: https://github.com/llvm/llvm-project/commit/2cb8d5ca3ad01c71b6481bd09779b4e82ccf8196.diff

LOG: [Pseudo Probe] Do not place functions in nodeduplicate COMDATs

For a function not in an IR COMDAT, currently we place it into a nodeduplicate IR
COMDAT so that its text section and its associated .pseudo_probe section will be
in the same section group, which can be retained or discarded by the linker as a
unit. However, the section group wastes space.

After D153189 uses SHF_LINK_ORDER to ensure a .pseudo_probe section will be
discarded when its associated text section is discarded, we can remove the
nodeduplicate IR change.

In the following example, the .pseudo_probe associated with .text.f is discarded as expected.
```
clang -c -ffunction-sections -fpseudo-probe-for-profiling -xc =(printf 'void _start(){} void f(){}') -o a.o
ld.lld --gc-sections --print-gc-sections a.o
```

Reviewed By: hoy

Differential Revision: https://reviews.llvm.org/D153191

Added: 
    

Modified: 
    llvm/lib/Transforms/IPO/SampleProfileProbe.cpp
    llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/IPO/SampleProfileProbe.cpp b/llvm/lib/Transforms/IPO/SampleProfileProbe.cpp
index 6ca794f9d38d4..0a42de7224b47 100644
--- a/llvm/lib/Transforms/IPO/SampleProfileProbe.cpp
+++ b/llvm/lib/Transforms/IPO/SampleProfileProbe.cpp
@@ -350,24 +350,6 @@ void SampleProfileProber::instrumentOneFunc(Function &F, TargetMachine *TM) {
   auto *NMD = M->getNamedMetadata(PseudoProbeDescMetadataName);
   assert(NMD && "llvm.pseudo_probe_desc should be pre-created");
   NMD->addOperand(MD);
-
-  // Preserve a comdat group to hold all probes materialized later. This
-  // allows that when the function is considered dead and removed, the
-  // materialized probes are disposed too.
-  // Imported functions are defined in another module. They do not need
-  // the following handling since same care will be taken for them in their
-  // original module. The pseudo probes inserted into an imported functions
-  // above will naturally not be emitted since the imported function is free
-  // from object emission. However they will be emitted together with the
-  // inliner functions that the imported function is inlined into. We are not
-  // creating a comdat group for an import function since it's useless anyway.
-  if (!F.isDeclarationForLinker()) {
-    if (TM) {
-      auto Triple = TM->getTargetTriple();
-      if (Triple.supportsCOMDAT() && TM->getFunctionSections())
-        getOrCreateFunctionComdat(F, Triple);
-    }
-  }
 }
 
 PreservedAnalyses SampleProfileProbePass::run(Module &M,

diff  --git a/llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll b/llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll
index faf770fe90416..13cfd820ae82c 100644
--- a/llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll
+++ b/llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll
@@ -19,6 +19,7 @@
 define void @foo(i32 %x) !dbg !3 {
 bb0:
   %cmp = icmp eq i32 %x, 0
+; CHECK-IL-LABEL: void @foo(i32 %x) !dbg ![[#]] {
 ; CHECK-IL: call void @llvm.pseudoprobe(i64 [[#GUID:]], i64 1, i32 0, i64 -1), !dbg ![[#FAKELINE:]]
 ; CHECK-MIR: PSEUDO_PROBE [[#GUID:]], 1, 0, 0
 ; CHECK-ASM: .pseudoprobe	[[#GUID:]] 1 0 0 foo
@@ -51,6 +52,7 @@ declare void @bar(i32 %x)
 
 define internal void @foo2(ptr %f) !dbg !4 {
 entry:
+; CHECK-IL-LABEL: void @foo2(ptr %f) !dbg ![[#]] {
 ; CHECK-IL: call void @llvm.pseudoprobe(i64 [[#GUID2:]], i64 1, i32 0, i64 -1)
 ; CHECK-MIR: PSEUDO_PROBE [[#GUID2:]], 1, 0, 0
 ; CHECK-ASM: .pseudoprobe	[[#GUID2:]] 1 0 0 foo2