[BPF] Use SectionForGlobal() for section names computation in BTF

[eddyz87]

Summary

Use function TargetLoweringObjectFile::SectionForGlobal() to compute
section names for globals described in BTF_KIND_DATASEC records.

This fixes a discrepancy in section name computation between
BTFDebug::processGlobals and the rest of the LLVM pipeline.

Specifically, the following example illustrates the discrepancy
before this commit:

struct Foo {
  int i;
} __attribute__((aligned(16)));
struct Foo foo = { 0 };

The initializer for 'foo' looks as follows:

%struct.Foo { i32 0, [12 x i8] undef }

TargetLoweringObjectFile::SectionForGlobal() classifies 'foo' as
a part of '.bss' section, while BTFDebug::processGlobals
classified it as a part of '.data' section because of the
following expression:

SecName = Global.getInitializer()->isZeroValue() ? ".bss" : ".data"

The isZeroValue() returns false because of the undef tail of the
initializer, while SectionForGlobal() allows such patterns in '.bss'.

[yonghong-song]

Otherwise, the patch LGTM.

llvm/lib/Target/BPF/BTFDebug.cpp

1383

Could you change the above 'auto' to actual types? Typically we use 'auto' for some *obvious* times. The above two are not obvious.

[yonghong-song]

llvm/test/CodeGen/BPF/BTF/global-var-bss-and-data.ll

2

The default check-prefix is 'CHECK' so '-check-prefixes=CHECK' can be rmoved.

15

Let us remove the above line 12-15. The 'llc' command line already has -march=<> flag. Also, the above target datalayout is only for little endian. The big endian datalayout is 'E-m:e-p:...'.

[eddyz87]

Styling updates as requested by Yonghong.

llvm/lib/Target/BPF/BTFDebug.cpp

 //===- BTFDebug.cpp - BTF Generator ---------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //

 }
 
 void BTFDebug::processGlobals(bool ProcessingMapDef) {
   // Collect all types referenced by globals.
   const Module *M = MMI->getModule();
   for (const GlobalVariable &Global : M->globals()) {
     // Decide the section name.
     StringRef SecName;
-    if (Global.hasSection()) {
-      SecName = Global.getSection();
-    } else if (Global.hasInitializer()) {
-      // data, bss, or readonly sections
-      if (Global.isConstant())
-        SecName = ".rodata";
-      else
-        SecName = Global.getInitializer()->isZeroValue() ? ".bss" : ".data";
+    Optional<SectionKind> GVKind;
+
+    if (!Global.isDeclarationForLinker())
+      GVKind = TargetLoweringObjectFile::getKindForGlobal(&Global, Asm->TM);
+
+    if (Global.isDeclarationForLinker())
+      SecName = Global.hasSection() ? Global.getSection() : "";
+    else if (GVKind->isCommon())
+      SecName = ".bss";
+    else {
+      auto *TLOF = Asm->TM.getObjFileLowering();
+      auto *Sec = TLOF->SectionForGlobal(&Global, Asm->TM);
+      SecName = Sec->getName();
     }
 
     if (ProcessingMapDef != SecName.startswith(".maps"))
       continue;
 
     // Create a .rodata datasec if the global variable is an initialized
     // constant with private linkage and if it won't be in .rodata.str<#>
     // and .rodata.cst<#> sections.
     if (SecName == ".rodata" && Global.hasPrivateLinkage() &&
         DataSecEntries.find(std::string(SecName)) == DataSecEntries.end()) {
-      SectionKind GVKind =
-          TargetLoweringObjectFile::getKindForGlobal(&Global, Asm->TM);
       // skip .rodata.str<#> and .rodata.cst<#> sections
-      if (!GVKind.isMergeableCString() && !GVKind.isMergeableConst()) {
+      if (!GVKind->isMergeableCString() && !GVKind->isMergeableConst()) {
         DataSecEntries[std::string(SecName)] =
             std::make_unique<BTFKindDataSec>(Asm, std::string(SecName));
       }
     }
 
     SmallVector<DIGlobalVariableExpression *, 1> GVs;
     Global.getDebugInfo(GVs);
 

llvm/test/CodeGen/BPF/BTF/global-var-bss-and-data.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+
+; Source code:
+;   struct s { int i; } __attribute__((aligned(16)));
+;   struct s a;          // .bss
+;   struct s b = { 0 };  // .bss
+;   struct s c = { 1 };  // .data
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm test.c
+
+; ModuleID = 'test.c'
+source_filename = "test.c"
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+target triple = "bpf"
+
+%struct.s = type { i32, [12 x i8] }
+
+@a = dso_local local_unnamed_addr global %struct.s zeroinitializer, align 16, !dbg !11
+@b = dso_local local_unnamed_addr global %struct.s { i32 0, [12 x i8] undef }, align 16, !dbg !0
+@c = dso_local local_unnamed_addr global %struct.s { i32 1, [12 x i8] undef }, align 16, !dbg !5
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!13, !14, !15, !16}
+!llvm.ident = !{!17}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 3, type: !7, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C11, file: !3, producer: "clang version 16.0.0 (https://github.com/llvm/llvm-project.git 3191e8e19f1a7007ddd0e55cee60a51a058c99f5)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/home/eddy/work/tmp", checksumkind: CSK_MD5, checksum: "b9d0621d30812c09bd3c6894f89ff5e4")
+!4 = !{!0, !5, !11}
+!5 = !DIGlobalVariableExpression(var: !6, expr: !DIExpression())
+!6 = distinct !DIGlobalVariable(name: "c", scope: !2, file: !3, line: 4, type: !7, isLocal: false, isDefinition: true)
+!7 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s", file: !3, line: 1, size: 128, align: 128, elements: !8)
+!8 = !{!9}
+!9 = !DIDerivedType(tag: DW_TAG_member, name: "i", scope: !7, file: !3, line: 1, baseType: !10, size: 32)
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DIGlobalVariableExpression(var: !12, expr: !DIExpression())
+!12 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 2, type: !7, isLocal: false, isDefinition: true)
+!13 = !{i32 7, !"Dwarf Version", i32 5}
+!14 = !{i32 2, !"Debug Info Version", i32 3}
+!15 = !{i32 1, !"wchar_size", i32 4}
+!16 = !{i32 7, !"frame-pointer", i32 2}
+!17 = !{!"clang version 16.0.0 (https://github.com/llvm/llvm-project.git 3191e8e19f1a7007ddd0e55cee60a51a058c99f5)"}
+
+; CHECK:         .section        .BTF,"",@progbits
+; CHECK-NEXT:    .short  60319                           # 0xeb9f
+; CHECK-NEXT:    .byte   1
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .long   24
+; CHECK-NEXT:    .long   0
+; CHECK-NEXT:    .long   148
+; CHECK-NEXT:    .long   148
+; CHECK-NEXT:    .long   26
+; CHECK-NEXT:    .long   1                               # BTF_KIND_STRUCT(id = 1)
+; CHECK-NEXT:    .long   67108865                        # 0x4000001
+; CHECK-NEXT:    .long   16
+; CHECK-NEXT:    .long   3
+; CHECK-NEXT:    .long   2
+; CHECK-NEXT:    .long   0                               # 0x0
+; CHECK-NEXT:    .long   5                               # BTF_KIND_INT(id = 2)
+; CHECK-NEXT:    .long   16777216                        # 0x1000000
+; CHECK-NEXT:    .long   4
+; CHECK-NEXT:    .long   16777248                        # 0x1000020
+; CHECK-NEXT:    .long   9                               # BTF_KIND_VAR(id = 3)
+; CHECK-NEXT:    .long   234881024                       # 0xe000000
+; CHECK-NEXT:    .long   1
+; CHECK-NEXT:    .long   1
+; CHECK-NEXT:    .long   11                              # BTF_KIND_VAR(id = 4)
+; CHECK-NEXT:    .long   234881024                       # 0xe000000
+; CHECK-NEXT:    .long   1
+; CHECK-NEXT:    .long   1
+; CHECK-NEXT:    .long   13                              # BTF_KIND_VAR(id = 5)
+; CHECK-NEXT:    .long   234881024                       # 0xe000000
+; CHECK-NEXT:    .long   1
+; CHECK-NEXT:    .long   1
+; CHECK-NEXT:    .long   15                              # BTF_KIND_DATASEC(id = 6)
+; CHECK-NEXT:    .long   251658242                       # 0xf000002
+; CHECK-NEXT:    .long   0
+; CHECK-NEXT:    .long   3
+; CHECK-NEXT:    .long   a
+; CHECK-NEXT:    .long   16
+; CHECK-NEXT:    .long   4
+; CHECK-NEXT:    .long   b
+; CHECK-NEXT:    .long   16
+; CHECK-NEXT:    .long   20                              # BTF_KIND_DATASEC(id = 7)
+; CHECK-NEXT:    .long   251658241                       # 0xf000001
+; CHECK-NEXT:    .long   0
+; CHECK-NEXT:    .long   5
+; CHECK-NEXT:    .long   c
+; CHECK-NEXT:    .long   16
+; CHECK-NEXT:    .byte   0                               # string offset=0
+; CHECK-NEXT:    .byte   115                             # string offset=1
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .byte   105                             # string offset=3
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .ascii  "int"                           # string offset=5
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .byte   97                              # string offset=9
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .byte   98                              # string offset=11
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .byte   99                              # string offset=13
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .ascii  ".bss"                          # string offset=15
+; CHECK-NEXT:    .byte   0
+; CHECK-NEXT:    .ascii  ".data"                         # string offset=20
+; CHECK-NEXT:    .byte   0