Add support for #pragma clang section

[javed.absar]

Summary

This is the corresponding llvm part for the clang patch https://reviews.llvm.org/D33412 that is also under review.

This patch adds support for a '#pragma clang section' directive in clang. An RFC was sent out earlier by my colleague James Molloy: http://lists.llvm.org/pipermail/cfe-dev/2017-March/053100.html

Purpose:
The purpose of this is to provide to developers a means to specify section-names for global variables, functions and static variables, using #pragma directives. This will provide a migration path towards clang for developers in the embedded and automotive domain. For example, AUTOSAR, an automotive standard, mandates the use of a #pragma in header files to determine in which sections initialised and uninitialized data get put into.
This feature is implemented in our legacy ARM Compiler 5 toolchain and GCC forks used across the automotive space that have this feature implemented compatible with the ARM Compiler 5 implementation. The documentation is here:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0472m/chr1359124985290.html

User-Visible Behavior:
The developer can specify section names as:

#pragma clang section bss="myBSS" data="myData" rodata="myRodata" text="myText"

The developer can "unspecify" a section name with empty string e.g.

#pragma clang section bss="" data="" text="" rodata=""

1. The pragma applies to all global variable, statics and function declarations from the pragma to the end of the translation unit.
2. The pragma clang section is enabled automatically, without need of any flags.
3. This feature is only defined to work sensibly for ELF targets.
4. If section name is specified through _attribute_((section("myname"))), then the attribute name gains precedence over any applicable section name via pragma directives.
5. Global variables, including - basic types, arrays, struct - that are initialized to zero e.g. int x = 0; will be placed in the named bss section, if one is present.
6. The section type using '#pragma clang section' approach does not does NOT try to infer section-kind from the name. For example, assigning a section ".bss.mysec" does NOT mean it will be placed in BSS.

Design:
The decision about which section-kind applies to each global is not taken by this implementation but is purely inquired from the default back-end implementation in LLVM. Once the section-kind is known, appropriate section name as specified by the user using pragma directive, is applied to that global. Note that this is more effective approach than choosing the section name in the front-end when optimisation passes have not been run and the final proper section is not known.

[javed.absar]

ping!

[rogfer01]

This supporting LLVM change looks good to me.

[rogfer01]

This looks good to me.

@rnk @jroelofs what do you think? Thanks!

[rogfer01]

llvm/trunk/include/llvm/IR/GlobalVariable.h

AttributeList getAttributesAsList(unsigned index) const {
     return AttributeList::get(getContext(), AS);
   }
 
   /// Set attribute list for this global
   void setAttributes(AttributeSet A) {
     Attrs = A;
   }
 
+  /// Check if section name is present
+  bool hasImplicitSection() const {
+    return getAttributes().hasAttribute("bss-section") ||
+           getAttributes().hasAttribute("data-section") ||
+           getAttributes().hasAttribute("rodata-section");
+  }
+
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const Value *V) {
     return V->getValueID() == Value::GlobalVariableVal;
   }
 };
 
 template <>
 struct OperandTraits<GlobalVariable> :
   public OptionalOperandTraits<GlobalVariable> {
 };
 
 DEFINE_TRANSPARENT_OPERAND_ACCESSORS(GlobalVariable, Value)
 
 } // end namespace llvm
 
 #endif // LLVM_IR_GLOBALVARIABLE_H

llvm/trunk/lib/CodeGen/GlobalMerge.cpp

bool GlobalMerge::doInitialization(Module &M) {
   DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
                                                         BSSGlobals;
   bool Changed = false;
   setMustKeepGlobalVariables(M);
 
   // Grab all non-const globals.
   for (auto &GV : M.globals()) {
     // Merge is safe for "normal" internal or external globals only
-    if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasSection())
+    if (GV.isDeclaration() || GV.isThreadLocal() ||
+        GV.hasSection() || GV.hasImplicitSection())
       continue;
 
     // It's not safe to merge globals that may be preempted
     if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
       continue;
 
     if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
         !GV.hasInternalLinkage())

llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

static const MCSymbolELF *getAssociatedSymbol(const GlobalObject *GO,
   GlobalObject *OtherGO = dyn_cast<GlobalObject>(VM->getValue());
   return OtherGO ? dyn_cast<MCSymbolELF>(TM.getSymbol(OtherGO)) : nullptr;
 }
 
 MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   StringRef SectionName = GO->getSection();
 
+  // Check if '#pragma clang section' name is applicable.
+  // Note that pragma directive overrides -ffunction-section, -fdata-section
+  // and so section name is exactly as user specified and not uniqued.
+  const GlobalVariable *GV = dyn_cast<GlobalVariable>(GO);
+  if (GV && GV->hasImplicitSection()) {
+    auto Attrs = GV->getAttributes();
+    if (Attrs.hasAttribute("bss-section") && Kind.isBSS()) {
+      SectionName = Attrs.getAttribute("bss-section").getValueAsString();
+    } else if (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly()) {
+      SectionName = Attrs.getAttribute("rodata-section").getValueAsString();
+    } else if (Attrs.hasAttribute("data-section") && Kind.isData()) {
+      SectionName = Attrs.getAttribute("data-section").getValueAsString();
+    }
+  }
+  const Function *F = dyn_cast<Function>(GO);
+  if (F && F->hasFnAttribute("implicit-section-name")) {
+    SectionName = F->getFnAttribute("implicit-section-name").getValueAsString();
+  }
+
   // Infer section flags from the section name if we can.
   Kind = getELFKindForNamedSection(SectionName, Kind);
 
   StringRef Group = "";
   unsigned Flags = getELFSectionFlags(Kind);
   if (const Comdat *C = getELFComdat(GO)) {
     Group = C->getName();
     Flags |= ELF::SHF_GROUP;

llvm/trunk/lib/Target/TargetLoweringObjectFile.cpp

 /// variable or function definition.  This should not be passed external (or
 /// available externally) globals.
 MCSection *TargetLoweringObjectFile::SectionForGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   // Select section name.
   if (GO->hasSection())
     return getExplicitSectionGlobal(GO, Kind, TM);
 
+  if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
+    auto Attrs = GVar->getAttributes();
+    if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) ||
+        (Attrs.hasAttribute("data-section") && Kind.isData()) ||
+        (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly()))  {
+       return getExplicitSectionGlobal(GO, Kind, TM);
+    }
+  }
+
+  if (auto *F = dyn_cast<Function>(GO)) {
+    if (F->hasFnAttribute("implicit-section-name"))
+      return getExplicitSectionGlobal(GO, Kind, TM);
+  }
+
   // Use default section depending on the 'type' of global
   return SelectSectionForGlobal(GO, Kind, TM);
 }
 
 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
     const Function &F, const TargetMachine &TM) const {
   unsigned Align = 0;
   return getSectionForConstant(F.getParent()->getDataLayout(),

llvm/trunk/test/CodeGen/ARM/clang-section.ll

+;RUN: llc -mtriple=armv7-eabi %s -o - | FileCheck %s
+;Test that global variables and functions are assigned to correct sections.
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7-arm-none-eabi"
+
+@a = global i32 0, align 4 #0
+@b = global i32 1, align 4 #0
+@c = global [4 x i32] zeroinitializer, align 4 #0
+@d = global [5 x i16] zeroinitializer, align 2 #0
+@e = global [6 x i16] [i16 0, i16 0, i16 1, i16 0, i16 0, i16 0], align 2 #0
+@f = constant i32 2, align 4 #0
+@h = global i32 0, align 4 #1
+@i = global i32 0, align 4 #2
+@j = constant i32 4, align 4 #2
+@k = global i32 0, align 4 #2
+@_ZZ3gooE7lstat_h = internal global i32 0, align 4 #2
+@_ZL1g = internal global [2 x i32] zeroinitializer, align 4 #0
+@l = global i32 5, align 4 #3
+@m = constant i32 6, align 4 #3
+@n = global i32 0, align 4
+@o = global i32 6, align 4
+@p = constant i32 7, align 4
+
+; Function Attrs: noinline nounwind
+define i32 @foo() #4 {
+entry:
+  %0 = load i32, i32* @b, align 4
+  ret i32 %0
+}
+
+; Function Attrs: noinline
+define i32 @goo() #5 {
+entry:
+  %call = call i32 @zoo(i32* getelementptr inbounds ([2 x i32], [2 x i32]* @_ZL1g, i32 0, i32 0), i32* @_ZZ3gooE7lstat_h)
+  ret i32 %call
+}
+
+declare i32 @zoo(i32*, i32*) #6
+
+; Function Attrs: noinline nounwind
+define i32 @hoo() #7 {
+entry:
+  %0 = load i32, i32* @b, align 4
+  ret i32 %0
+}
+
+attributes #0 = { "bss-section"="my_bss.1" "data-section"="my_data.1" "rodata-section"="my_rodata.1" }
+attributes #1 = { "data-section"="my_data.1" "rodata-section"="my_rodata.1" }
+attributes #2 = { "bss-section"="my_bss.2" "rodata-section"="my_rodata.1" }
+attributes #3 = { "bss-section"="my_bss.2" "data-section"="my_data.2" "rodata-section"="my_rodata.2" }
+attributes #4 = { noinline nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "denormal-fp-math"="preserve-sign" "disable-tail-calls"="false" "implicit-section-name"="my_text.1" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="cortex-a9" "target-features"="+dsp,+fp16,+neon,+vfp3" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #5 = { noinline "correctly-rounded-divide-sqrt-fp-math"="false" "denormal-fp-math"="preserve-sign" "disable-tail-calls"="false" "implicit-section-name"="my_text.2" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="cortex-a9" "target-features"="+dsp,+fp16,+neon,+vfp3" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #6 = { "correctly-rounded-divide-sqrt-fp-math"="false" "denormal-fp-math"="preserve-sign" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="true" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="cortex-a9" "target-features"="+dsp,+fp16,+neon,+vfp3" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #7 = { noinline nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "denormal-fp-math"="preserve-sign" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="cortex-a9" "target-features"="+dsp,+fp16,+neon,+vfp3" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.module.flags = !{!0, !1, !2, !3}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{i32 1, !"static_rwdata", i32 1}
+!2 = !{i32 1, !"enumsize_buildattr", i32 2}
+!3 = !{i32 1, !"armlib_unavailable", i32 0}
+
+;CHECK:   .section  my_text.1,"ax",%progbits
+;CHECK:   .type foo,%function
+;CHECK: foo:
+
+;CHECK:   .section  my_text.2,"ax",%progbits
+;CHECK:   .type goo,%function
+;CHECK: goo:
+
+;CHECK:   .text
+;CHECK:   .type hoo,%function
+;CHECK: hoo:
+
+;CHECK:   .type a,%object
+;CHECK:   .section  my_bss.1,"aw",%nobits
+;CHECK: a:
+
+;CHECK:   .type b,%object
+;CHECK:   .section  my_data.1,"aw",%progbits
+;CHECK: b:
+
+;CHECK:   .type c,%object
+;CHECK:   .section  my_bss.1,"aw",%nobits
+;CHECK: c:
+
+;CHECK:   .type d,%object
+;CHECK: d:
+
+;CHECK:   .type e,%object
+;CHECK:   .section  my_data.1,"aw",%progbits
+;CHECK: e:
+
+;CHECK:   .type f,%object
+;CHECK:   .section  my_rodata.1,"a",%progbits
+;CHECK: f:
+
+;CHECK:   .type h,%object
+;CHECK:   .bss
+;CHECK: h:
+
+;CHECK:   .type i,%object
+;CHECK:   .section  my_bss.2,"aw",%nobits
+;CHECK: i:
+
+;CHECK:   .type j,%object
+;CHECK:   .section  my_rodata.1,"a",%progbits
+;CHECK: j:
+
+;CHECK:   .type k,%object
+;CHECK:   .section  my_bss.2,"aw",%nobits
+;CHECK: k:
+
+;CHECK:   .type _ZZ3gooE7lstat_h,%object @ @_ZZ3gooE7lstat_h
+;CHECK: _ZZ3gooE7lstat_h:
+
+;CHECK:   .type _ZL1g,%object
+;CHECK:   .section  my_bss.1,"aw",%nobits
+;CHECK: _ZL1g:
+
+;CHECK:   .type l,%object
+;CHECK:   .section  my_data.2,"aw",%progbits
+;CHECK: l:
+
+;CHECK:   .type m,%object
+;CHECK:   .section  my_rodata.2,"a",%progbits
+;CHECK: m:
+
+;CHECK:   .type n,%object
+;CHECK:   .bss
+;CHECK: n:
+
+;CHECK:   .type o,%object
+;CHECK:   .data
+;CHECK: o:
+
+;CHECK:   .type p,%object
+;CHECK:   .section  .rodata,"a",%progbits
+;CHECK: p:

llvm/trunk/test/MC/ELF/clang-section.s

+// RUN: llvm-mc -filetype=obj -triple arm-eabi %s -o - | llvm-readobj -s -t | FileCheck %s
+// Test that global variables and functions are assigned correct section.
+  .text
+  .syntax unified
+  .eabi_attribute 67, "2.09"  @ Tag_conformance
+  .eabi_attribute 6, 1  @ Tag_CPU_arch
+  .eabi_attribute 8, 1  @ Tag_ARM_ISA_use
+  .eabi_attribute 17, 1 @ Tag_ABI_PCS_GOT_use
+  .eabi_attribute 20, 1 @ Tag_ABI_FP_denormal
+  .eabi_attribute 21, 1 @ Tag_ABI_FP_exceptions
+  .eabi_attribute 23, 3 @ Tag_ABI_FP_number_model
+  .eabi_attribute 34, 1 @ Tag_CPU_unaligned_access
+  .eabi_attribute 24, 1 @ Tag_ABI_align_needed
+  .eabi_attribute 25, 1 @ Tag_ABI_align_preserved
+  .eabi_attribute 38, 1 @ Tag_ABI_FP_16bit_format
+  .eabi_attribute 18, 4 @ Tag_ABI_PCS_wchar_t
+  .eabi_attribute 26, 2 @ Tag_ABI_enum_size
+  .eabi_attribute 14, 0 @ Tag_ABI_PCS_R9_use
+  .section  my_text.1,"ax",%progbits
+  .globl  foo
+  .p2align  2
+  .type foo,%function
+  .code 32                      @ @foo
+foo:
+  .fnstart
+@ BB#0:                                 @ %entry
+  ldr r0, .LCPI0_0
+  ldr r0, [r0]
+  mov pc, lr
+  .p2align  2
+@ BB#1:
+.LCPI0_0:
+  .long b
+.Lfunc_end0:
+  .size foo, .Lfunc_end0-foo
+  .cantunwind
+  .fnend
+
+  .section  my_text.2,"ax",%progbits
+  .globl  goo
+  .p2align  2
+  .type goo,%function
+  .code 32                      @ @goo
+goo:
+  .fnstart
+@ BB#0:                                 @ %entry
+  .save {r11, lr}
+  push  {r11, lr}
+  ldr r0, .LCPI1_0
+  ldr r1, .LCPI1_1
+  bl  zoo
+  pop {r11, lr}
+  mov pc, lr
+  .p2align  2
+@ BB#1:
+.LCPI1_0:
+  .long _ZL1g
+.LCPI1_1:
+  .long _ZZ3gooE7lstat_h
+.Lfunc_end1:
+  .size goo, .Lfunc_end1-goo
+  .cantunwind
+  .fnend
+
+  .text
+  .globl  hoo
+  .p2align  2
+  .type hoo,%function
+  .code 32                      @ @hoo
+hoo:
+  .fnstart
+@ BB#0:                                 @ %entry
+  ldr r0, .LCPI2_0
+  ldr r0, [r0]
+  mov pc, lr
+  .p2align  2
+@ BB#1:
+.LCPI2_0:
+  .long b
+.Lfunc_end2:
+  .size hoo, .Lfunc_end2-hoo
+  .cantunwind
+  .fnend
+
+  .type a,%object               @ @a
+  .section  my_bss.1,"aw",%nobits
+  .globl  a
+  .p2align  2
+a:
+  .long 0                       @ 0x0
+  .size a, 4
+
+  .type b,%object               @ @b
+  .section  my_data.1,"aw",%progbits
+  .globl  b
+  .p2align  2
+b:
+  .long 1                       @ 0x1
+  .size b, 4
+
+  .type c,%object               @ @c
+  .section  my_bss.1,"aw",%nobits
+  .globl  c
+  .p2align  2
+c:
+  .zero 16
+  .size c, 16
+
+  .type d,%object               @ @d
+  .globl  d
+  .p2align  1
+d:
+  .zero 10
+  .size d, 10
+
+  .type e,%object               @ @e
+  .section  my_data.1,"aw",%progbits
+  .globl  e
+  .p2align  1
+e:
+  .short  0                       @ 0x0
+  .short  0                       @ 0x0
+  .short  1                       @ 0x1
+  .short  0                       @ 0x0
+  .short  0                       @ 0x0
+  .short  0                       @ 0x0
+  .size e, 12
+
+  .type f,%object               @ @f
+  .section  my_rodata.1,"a",%progbits
+  .globl  f
+  .p2align  2
+f:
+  .long 2                       @ 0x2
+  .size f, 4
+
+  .type h,%object               @ @h
+  .bss
+  .globl  h
+  .p2align  2
+h:
+  .long 0                       @ 0x0
+  .size h, 4
+
+  .type i,%object               @ @i
+  .section  my_bss.2,"aw",%nobits
+  .globl  i
+  .p2align  2
+i:
+  .long 0                       @ 0x0
+  .size i, 4
+
+  .type j,%object               @ @j
+  .section  my_rodata.1,"a",%progbits
+  .globl  j
+  .p2align  2
+j:
+  .long 4                       @ 0x4
+  .size j, 4
+
+  .type k,%object               @ @k
+  .section  my_bss.2,"aw",%nobits
+  .globl  k
+  .p2align  2
+k:
+  .long 0                       @ 0x0
+  .size k, 4
+
+  .type _ZZ3gooE7lstat_h,%object @ @_ZZ3gooE7lstat_h
+  .p2align  2
+_ZZ3gooE7lstat_h:
+  .long 0                       @ 0x0
+  .size _ZZ3gooE7lstat_h, 4
+
+  .type _ZL1g,%object           @ @_ZL1g
+  .section  my_bss.1,"aw",%nobits
+  .p2align  2
+_ZL1g:
+  .zero 8
+  .size _ZL1g, 8
+
+  .type l,%object               @ @l
+  .section  my_data.2,"aw",%progbits
+  .globl  l
+  .p2align  2
+l:
+  .long 5                       @ 0x5
+  .size l, 4
+
+  .type m,%object               @ @m
+  .section  my_rodata.2,"a",%progbits
+  .globl  m
+  .p2align  2
+m:
+  .long 6                       @ 0x6
+  .size m, 4
+
+  .type n,%object               @ @n
+  .bss
+  .globl  n
+  .p2align  2
+n:
+  .long 0                       @ 0x0
+  .size n, 4
+
+  .type o,%object               @ @o
+  .data
+  .globl  o
+  .p2align  2
+o:
+  .long 6                       @ 0x6
+  .size o, 4
+
+  .type p,%object               @ @p
+  .section  .rodata,"a",%progbits
+  .globl  p
+  .p2align  2
+p:
+  .long 7                       @ 0x7
+  .size p, 4
+
+
+  .ident  "clang version 5.0.0 (http://llvm.org/git/clang.git 254242a3ad440307fb451093a429c71ea9a8c888) (http://llvm.org/git/llvm.git 3c8daefbe3d1672ac1dae775b211f881f0063038)"
+  .section  ".note.GNU-stack","",%progbits
+  .eabi_attribute 30, 1 @ Tag_ABI_optimization_goals
+
+//CHECK:   Section {
+//CHECK:     Name: .text
+//CHECK:     Type: SHT_PROGBITS (0x1)
+//CHECK:     Flags [ (0x6)
+//CHECK:       SHF_ALLOC (0x2)
+//CHECK:       SHF_EXECINSTR (0x4)
+//CHECK:     ]
+//CHECK:   }
+//CHECK:   Section {
+//CHECK:     Name: my_text.1
+//CHECK:     Type: SHT_PROGBITS (0x1)
+//CHECK:     Flags [ (0x6)
+//CHECK:       SHF_ALLOC (0x2)
+//CHECK:       SHF_EXECINSTR (0x4)
+//CHECK:     ]
+//CHECK:   }
+//CHECK:   Section {
+//CHECK:     Name: my_text.2
+//CHECK:     Type: SHT_PROGBITS (0x1)
+//CHECK:     Flags [ (0x6)
+//CHECK:       SHF_ALLOC (0x2)
+//CHECK:       SHF_EXECINSTR (0x4)
+//CHECK:     ]
+//CHECK:   }
+//CHECK:   Section {
+//CHECK:     Name: my_bss.1
+//CHECK:     Type: SHT_NOBITS (0x8)
+//CHECK:     Flags [ (0x3)
+//CHECK:       SHF_ALLOC (0x2)
+//CHECK:       SHF_WRITE (0x1)
+//CHECK:     ]
+//CHECK:   }
+//CHECK:   Section {
+//CHECK:     Name: my_data.1
+//CHECK:     Type: SHT_PROGBITS (0x1)
+//CHECK:     Flags [ (0x3)
+//CHECK:       SHF_ALLOC (0x2)
+//CHECK:       SHF_WRITE (0x1)
+//CHECK:     ]
+//CHECK:   }
+//CHECK:   Section {
+//CHECK:     Name: my_rodata.1
+//CHECK:     Type: SHT_PROGBITS (0x1)
+//CHECK:     Flags [ (0x2)
+//CHECK:       SHF_ALLOC (0x2)
+//CHECK:     ]
+//CHECK:   }
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