[llvm] [X86] Set .llvmbc and .llvmcmd to exclude sections (PR #151910)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lto

Author: Haohai Wen (HaohaiWen)

<details>
<summary>Changes</summary>

The linked .llvmbc and .llvmcmd sections are meaningless in the final
binary. They can occupy significant space and should be simply discarded.
This patch sets them to exclude section kind for ELF and COFF. Wasm does
not support exclude kind, so those sections are retained as metadata and
they will be dropped by linker explicitly.

---
Full diff: https://github.com/llvm/llvm-project/pull/151910.diff


3 Files Affected:

- (modified) llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp (+9-4) 
- (modified) llvm/test/CodeGen/X86/embed-bitcode.ll (+4-2) 
- (modified) llvm/test/LTO/X86/embed-bitcode.ll (+1-1) 


``````````diff
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index d19ef923ef740..1220bed503fc1 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -466,10 +466,12 @@ static SectionKind getELFKindForNamedSection(StringRef Name, SectionKind K) {
       Name == getInstrProfSectionName(IPSK_covdata, Triple::ELF,
                                       /*AddSegmentInfo=*/false) ||
       Name == getInstrProfSectionName(IPSK_covname, Triple::ELF,
-                                      /*AddSegmentInfo=*/false) ||
-      Name == ".llvmbc" || Name == ".llvmcmd")
+                                      /*AddSegmentInfo=*/false))
     return SectionKind::getMetadata();
 
+  if (Name == ".llvmbc" || Name == ".llvmcmd")
+    return SectionKind::getExclude();
+
   if (!Name.starts_with(".")) return K;
 
   // Default implementation based on some magic section names.
@@ -1735,9 +1737,12 @@ MCSection *TargetLoweringObjectFileCOFF::getExplicitSectionGlobal(
       Name == getInstrProfSectionName(IPSK_covdata, Triple::COFF,
                                       /*AddSegmentInfo=*/false) ||
       Name == getInstrProfSectionName(IPSK_covname, Triple::COFF,
-                                      /*AddSegmentInfo=*/false) ||
-      Name == ".llvmbc" || Name == ".llvmcmd")
+                                      /*AddSegmentInfo=*/false))
     Kind = SectionKind::getMetadata();
+
+  if (Name == ".llvmbc" || Name == ".llvmcmd")
+    Kind = SectionKind::getExclude();
+
   int Selection = 0;
   unsigned Characteristics = getCOFFSectionFlags(Kind, TM);
   StringRef COMDATSymName = "";
diff --git a/llvm/test/CodeGen/X86/embed-bitcode.ll b/llvm/test/CodeGen/X86/embed-bitcode.ll
index d4af9544bc1be..7b08c69926d8a 100644
--- a/llvm/test/CodeGen/X86/embed-bitcode.ll
+++ b/llvm/test/CodeGen/X86/embed-bitcode.ll
@@ -4,17 +4,19 @@
 ; RUN: llvm-readobj -S %t | FileCheck %s --check-prefix=COFF
 
 ; CHECK:      .text    PROGBITS 0000000000000000 [[#%x,OFF:]] 000000 00 AX 0
-; CHECK-NEXT: .llvmbc  PROGBITS 0000000000000000 [[#%x,OFF:]] 000004 00    0
-; CHECK-NEXT: .llvmcmd PROGBITS 0000000000000000 [[#%x,OFF:]] 000005 00    0
+; CHECK-NEXT: .llvmbc  PROGBITS 0000000000000000 [[#%x,OFF:]] 000004 00 E  0
+; CHECK-NEXT: .llvmcmd PROGBITS 0000000000000000 [[#%x,OFF:]] 000005 00 E  0
 
 ; COFF:      Name: .llvmbc (2E 6C 6C 76 6D 62 63 00)
 ; COFF:      Characteristics [
 ; COFF-NEXT:   IMAGE_SCN_ALIGN_1BYTES
+; COFF-NEXT:   IMAGE_SCN_LNK_REMOVE
 ; COFF-NEXT:   IMAGE_SCN_MEM_DISCARDABLE
 ; COFF-NEXT: ]
 ; COFF:      Name: .llvmcmd (2E 6C 6C 76 6D 63 6D 64)
 ; COFF:      Characteristics [
 ; COFF-NEXT:   IMAGE_SCN_ALIGN_1BYTES
+; COFF-NEXT:   IMAGE_SCN_LNK_REMOVE
 ; COFF-NEXT:   IMAGE_SCN_MEM_DISCARDABLE
 ; COFF-NEXT: ]
 
diff --git a/llvm/test/LTO/X86/embed-bitcode.ll b/llvm/test/LTO/X86/embed-bitcode.ll
index bdddd079d2265..d8ebbdf85bc32 100644
--- a/llvm/test/LTO/X86/embed-bitcode.ll
+++ b/llvm/test/LTO/X86/embed-bitcode.ll
@@ -19,7 +19,7 @@
 ; RUN: llvm-dis %t-embedded.bc -o - | FileCheck %s --check-prefixes=CHECK-LL,CHECK-NOOPT
 
 ; CHECK-ELF:      .text   PROGBITS 0000000000000000 [[#%x,OFF:]] [[#%x,SIZE:]] 00 AX 0
-; CHECK-ELF-NEXT: .llvmbc PROGBITS 0000000000000000 [[#%x,OFF:]] [[#%x,SIZE:]] 00    0
+; CHECK-ELF-NEXT: .llvmbc PROGBITS 0000000000000000 [[#%x,OFF:]] [[#%x,SIZE:]] 00 E  0
 
 ; CHECK-LL: @_start
 ; CHECK-LL: @foo

``````````

</details>


https://github.com/llvm/llvm-project/pull/151910