[llvm] 6ae7b40 - Set alignment of .llvmbc and .llvmcmd to 1

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2020-08-29T18:27:34-07:00
New Revision: 6ae7b403c3e1aebcb825d3dd4777d3c1149d6d67

URL: https://github.com/llvm/llvm-project/commit/6ae7b403c3e1aebcb825d3dd4777d3c1149d6d67
DIFF: https://github.com/llvm/llvm-project/commit/6ae7b403c3e1aebcb825d3dd4777d3c1149d6d67.diff

LOG: Set alignment of .llvmbc and .llvmcmd to 1

Otherwise their alignment is dependent on the size of the section.  If the size
is large than 16, the alignment will be 16.

16 is a bad choice for both .llvmbc and .llvmcmd because the padding between two
contributions from input sections is of a variable size.

A bitstream is actually guaranteed to be 4-byte aligned, but consumers don't
need this property.

Added: 
    

Modified: 
    clang/test/Frontend/embed-bitcode.ll
    clang/test/Frontend/x86-embed-bitcode.ll
    llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
    llvm/test/CodeGen/X86/embed-bitcode.ll

Removed: 
    


################################################################################
diff  --git a/clang/test/Frontend/embed-bitcode.ll b/clang/test/Frontend/embed-bitcode.ll
index 75cdc5f657fc..defb2d12f2f0 100644
--- a/clang/test/Frontend/embed-bitcode.ll
+++ b/clang/test/Frontend/embed-bitcode.ll
@@ -43,9 +43,9 @@
 ; RUN:    | FileCheck %s -check-prefix=CHECK-WARNING
 
 ; CHECK-ELF: @llvm.embedded.module
-; CHECK-ELF: section ".llvmbc"
+; CHECK-ELF-SAME: section ".llvmbc", align 1
 ; CHECK-ELF: @llvm.cmdline
-; CHECK-ELF: section ".llvmcmd"
+; CHECK-ELF-SAME: section ".llvmcmd", align 1
 
 ; CHECK-ONLY-BITCODE: @llvm.embedded.module = private constant
 ; CHECK-ONLY-BITCODE: c"\DE\C0\17\0B

diff  --git a/clang/test/Frontend/x86-embed-bitcode.ll b/clang/test/Frontend/x86-embed-bitcode.ll
index 709f56bdfe81..b49fa6247c16 100644
--- a/clang/test/Frontend/x86-embed-bitcode.ll
+++ b/clang/test/Frontend/x86-embed-bitcode.ll
@@ -43,17 +43,17 @@
 ; CHECK: section "__LLVM,__cmdline"
 
 ; CHECK-ELF: @llvm.embedded.module
-; CHECK-ELF: section ".llvmbc"
+; CHECK-ELF-SAME: section ".llvmbc", align 1
 ; CHECK-ELF: @llvm.cmdline
-; CHECK-ELF: section ".llvmcmd"
+; CHECK-ELF-SAME: section ".llvmcmd", align 1
 
 ; CHECK-ELF-MARKER: @llvm.embedded.module
 ; CHECK-ELF-MARKER: constant [0 x i8] zeroinitializer
 ; CHECK-ELF-MARKER: @llvm.cmdline
-; CHECK-ELF-MARKER: section ".llvmcmd"
+; CHECK-ELF-MARKER: section ".llvmcmd", align 1
 
 ; CHECK-ELF-ONLY-BITCODE: @llvm.embedded.module
-; CHECK-ELF-ONLY-BITCODE: section ".llvmbc"
+; CHECK-ELF-ONLY-BITCODE-SAME: section ".llvmbc", align 1
 ; CHECK-ELF-ONLY-BITCODE-NOT: @llvm.cmdline
 ; CHECK-ELF-ONLY-BITCODE-NOT: section ".llvmcmd"
 

diff  --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index f2630903dba1..eaea026681b1 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -4852,6 +4852,9 @@ void llvm::EmbedBitcodeInModule(llvm::Module &M, llvm::MemoryBufferRef Buf,
       M, ModuleConstant->getType(), true, llvm::GlobalValue::PrivateLinkage,
       ModuleConstant);
   GV->setSection(getSectionNameForBitcode(T));
+  // Set alignment to 1 to prevent padding between two contributions from input
+  // sections after linking.
+  GV->setAlignment(Align(1));
   UsedArray.push_back(
       ConstantExpr::getPointerBitCastOrAddrSpaceCast(GV, UsedElementType));
   if (llvm::GlobalVariable *Old =
@@ -4875,6 +4878,7 @@ void llvm::EmbedBitcodeInModule(llvm::Module &M, llvm::MemoryBufferRef Buf,
                                   llvm::GlobalValue::PrivateLinkage,
                                   CmdConstant);
     GV->setSection(getSectionNameForCommandline(T));
+    GV->setAlignment(Align(1));
     UsedArray.push_back(
         ConstantExpr::getPointerBitCastOrAddrSpaceCast(GV, UsedElementType));
     if (llvm::GlobalVariable *Old = M.getGlobalVariable("llvm.cmdline", true)) {

diff  --git a/llvm/test/CodeGen/X86/embed-bitcode.ll b/llvm/test/CodeGen/X86/embed-bitcode.ll
index 00dd7ef17d56..8c56dfe2168f 100644
--- a/llvm/test/CodeGen/X86/embed-bitcode.ll
+++ b/llvm/test/CodeGen/X86/embed-bitcode.ll
@@ -5,6 +5,6 @@
 ; CHECK-NEXT: .llvmbc  PROGBITS 0000000000000000 [[#%x,OFF:]] 000004 00    0
 ; CHECK-NEXT: .llvmcmd PROGBITS 0000000000000000 [[#%x,OFF:]] 000005 00    0
 
- at llvm.embedded.module = private constant [4 x i8] c"BC\C0\DE", section ".llvmbc"
- at llvm.cmdline = private constant [5 x i8] c"-cc1\00", section ".llvmcmd"
+ at llvm.embedded.module = private constant [4 x i8] c"BC\C0\DE", section ".llvmbc", align 1
+ at llvm.cmdline = private constant [5 x i8] c"-cc1\00", section ".llvmcmd", align 1
 @llvm.compiler.used = appending global [2 x i8*] [i8* getelementptr inbounds ([4 x i8], [4 x i8]* @llvm.embedded.module, i32 0, i32 0), i8* getelementptr inbounds ([5 x i8], [5 x i8]* @llvm.cmdline, i32 0, i32 0)], section "llvm.metadata"