caio/clang-p2996
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Ensure that prefix data is preserved with subsections-via-symbols

Browse Source 
On MachO platforms that use subsections-via-symbols dead code stripping will
drop prefix data. Unfortunately there is no great way to convey the relationship
between a function and its prefix data to the linker. We are forced to use a bit
of a hack: we give the prefix data it’s own symbol, and mark the actual function
entry an .alt_entry.

Patch by Moritz Angermann!

Differential Revision: https://reviews.llvm.org/D30770

llvm-svn: 297804
This commit is contained in:
Peter Collingbourne
2017-03-15 04:18:16 +00:00
parent 3b4e29ba68
commit 7f6e2c97b8
3 changed files with 65 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

19
llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
View File

@@ -646,8 +646,23 @@ void AsmPrinter::EmitFunctionHeader() {
}
// Emit the prefix data.
if (F->hasPrefixData())
EmitGlobalConstant(F->getParent()->getDataLayout(), F->getPrefixData());
if (F->hasPrefixData()) {
if (MAI->hasSubsectionsViaSymbols()) {
// Preserving prefix data on platforms which use subsections-via-symbols
// is a bit tricky. Here we introduce a symbol for the prefix data
// and use the .alt_entry attribute to mark the function's real entry point
// as an alternative entry point to the prefix-data symbol.
MCSymbol *PrefixSym = OutContext.createLinkerPrivateTempSymbol();
OutStreamer->EmitLabel(PrefixSym);
EmitGlobalConstant(F->getParent()->getDataLayout(), F->getPrefixData());
// Emit an .alt_entry directive for the actual function symbol.
OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_AltEntry);
} else {
EmitGlobalConstant(F->getParent()->getDataLayout(), F->getPrefixData());
}
}
// Emit the CurrentFnSym. This is a virtual function to allow targets to
// do their wild and crazy things as required.

29
llvm/test/CodeGen/AArch64/prefixdata.ll Normal file
View File

@@ -0,0 +1,29 @@
; RUN: llc < %s -mtriple=aarch64-apple-darwin | FileCheck --check-prefix=MACHO %s
; RUN: llc < %s -mtriple=aarch64-pc-linux | FileCheck --check-prefix=ELF %s
@i = linkonce_odr global i32 1
; MACHO: ltmp0:
; MACHO-NEXT: .long 1
; MACHO-NEXT: .alt_entry _f
; MACHO-NEXT: _f:
; ELF: .type f,@function
; ELF-NEXT: .word 1
; ELF-NEXT: // 0x1
; ELF-NEXT: f:
define void @f() prefix i32 1 {
ret void
}
; MACHO: ltmp1:
; MACHO-NEXT: .quad _i
; MACHO-NEXT: .alt_entry _g
; MACHO-NEXT: _g:
; ELF: .type g,@function
; ELF-NEXT: .xword i
; ELF-NEXT: g:
define void @g() prefix i32* @i {
ret void
}
; MACHO: .subsections_via_symbols

27
llvm/test/CodeGen/X86/prefixdata.ll
View File

@@ -1,18 +1,29 @@
; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
; RUN: llc < %s -mtriple=x86_64-apple-darwin | FileCheck --check-prefix=MACHO %s
; RUN: llc < %s -mtriple=x86_64-pc-linux | FileCheck --check-prefix=ELF %s
@i = linkonce_odr global i32 1
; CHECK: .type f,@function
; CHECK-NEXT: .long 1
; CHECK-NEXT: # 0x1
; CHECK-NEXT: f:
; MACHO: ltmp0:
; MACHO-NEXT: .long 1
; MACHO-NEXT: .alt_entry _f
; MACHO-NEXT: _f:
; ELF: .type f,@function
; ELF-NEXT: .long 1
; ELF-NEXT: # 0x1
; ELF-NEXT: f:
define void @f() prefix i32 1 {
ret void
}
; CHECK: .type g,@function
; CHECK-NEXT: .quad i
; CHECK-NEXT: g:
; MACHO: ltmp1:
; MACHO-NEXT: .quad _i
; MACHO-NEXT: .alt_entry _g
; MACHO-NEXT: _g:
; ELF: .type g,@function
; ELF-NEXT: .quad i
; ELF-NEXT: g:
define void @g() prefix i32* @i {
ret void
}
; MACHO: .subsections_via_symbols