[PATCH] D48916: Fix setting of empty implicit-section-name attribute for functions affected by '#pragma clang section'

Petr Pavlu via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Jul 4 01:01:19 PDT 2018

petpav01 created this revision.
petpav01 added reviewers: javed.absar, chill, rnk.
Herald added subscribers: cfe-commits, kristof.beyls.

Code in `CodeGenModule::SetFunctionAttributes()` can set an empty attribute `implicit-section-name` on a function that is affected by `#pragma clang text="section"`. This is incorrect because the attribute should contain a valid section name. If the function additionally also uses `__attribute__((section("section")))` then this results in emitting the function in a section with an empty name.


  $ cat repr.c
  #pragma clang section text=".text_pragma"
  void f(void) __attribute__((section(".text_attr")));
  void f(void) {}
  $ clang -target arm-none-eabi -march=armv7-a -c repr.c
  $ llvm-objdump -h repr.o
  repr.o: file format ELF32-arm-little
  Idx Name          Size      Address          Type
    0               00000000 0000000000000000
    1 .strtab       0000005d 0000000000000000
    2 .text         00000000 0000000000000000 TEXT
    3               00000004 0000000000000000 TEXT
    4 .ARM.exidx    00000008 0000000000000000
    5 .rel.ARM.exidx 00000008 0000000000000000
    6 .comment      000000b2 0000000000000000
    7 .note.GNU-stack 00000000 0000000000000000
    8 .ARM.attributes 0000003a 0000000000000000
    9 .symtab       00000050 0000000000000000

Section with index 3 contains the code for function `f()`. The name of the section should be `.text_attr` but is instead empty.

The following happens in this example:

- `CodeGenModule::EmitGlobalFunctionDefinition()` is called to emit IR for `f()`. It invokes `GetAddrOfFunction()` -> `GetOrCreateLLVMFunction()` -> `SetFunctionAttributes()`. The last method notices that the `FunctionDecl` has the `PragmaClangTextSectionAttr` attribute and wrongly sets empty `implicit-section-name` attribute on the resulting `llvm::Function`.
- `EmitGlobalFunctionDefinition()` calls `setNonAliasAttributes()` which normally also sets the `implicit-section-name` attribute but because the `Decl` has `SectionAttr` it gets skipped.

The patch fixes the issue by removing the problematic code that sets empty `implicit-section-name` from `CodeGenModule::SetFunctionAttributes()`. The idea is that it is sufficient to set this attribute only from `setNonAliasAttributes()` when a function is emitted.

  rC Clang



Index: test/CodeGen/clang-sections-attribute.c
--- /dev/null
+++ test/CodeGen/clang-sections-attribute.c
@@ -0,0 +1,76 @@
+// RUN: %clang_cc1 -emit-llvm -triple arm-none-eabi -o - %s | FileCheck %s
+// Test interaction between __attribute__((section())) and '#pragma clang
+// section' directives. The attribute should always have higher priority than
+// the pragma.
+// Text tests.
+#pragma clang section text=".ext_fun_pragma"
+void ext_fun(void) __attribute__((section(".ext_fun_attr")));
+void ext_fun(void) {}
+#pragma clang section text=""
+void ext_fun2(void) __attribute__((section(".ext_fun2_attr")));
+#pragma clang section text=".ext_fun2_pragma"
+void ext_fun2(void) {}
+#pragma clang section text=""
+#pragma clang section text=".int_fun_pragma"
+static void int_fun(void) __attribute__((section(".int_fun_attr"), used));
+static void int_fun(void) {}
+#pragma clang section text=""
+static void int_fun2(void) __attribute__((section(".int_fun2_attr"), used));
+#pragma clang section text=".int_fun2_pragma"
+static void int_fun2(void) {}
+#pragma clang section text=""
+// Rodata tests.
+#pragma clang section rodata=".ext_const_pragma"
+const int ext_const = 1;
+#pragma clang section rodata=""
+#pragma clang section rodata=".int_const_pragma"
+__attribute__((section(".int_const_attr"), used))
+static const int int_const = 1;
+#pragma clang section rodata=""
+// Data tests.
+#pragma clang section data=".ext_var_pragma"
+int ext_var = 1;
+#pragma clang section data=""
+#pragma clang section data=".int_var_pragma"
+__attribute__((section(".int_var_attr"), used))
+static int int_var = 1;
+#pragma clang section data=""
+// Bss tests.
+#pragma clang section bss=".ext_zvar_pragma"
+int ext_zvar;
+#pragma clang section bss=""
+#pragma clang section bss=".int_zvar_pragma"
+__attribute__((section(".int_zvar_attr"), used))
+static int int_zvar;
+#pragma clang section bss=""
+// CHECK: @ext_const = constant i32 1, section ".ext_const_attr", align 4{{$}}
+// CHECK: @int_const = internal constant i32 1, section ".int_const_attr", align 4{{$}}
+// CHECK: @ext_var = global i32 1, section ".ext_var_attr", align 4{{$}}
+// CHECK: @int_var = internal global i32 1, section ".int_var_attr", align 4{{$}}
+// CHECK: @ext_zvar = global i32 0, section ".ext_zvar_attr", align 4{{$}}
+// CHECK: @int_zvar = internal global i32 0, section ".int_zvar_attr", align 4{{$}}
+// CHECK: define void @ext_fun() #0 section ".ext_fun_attr"
+// CHECK: define void @ext_fun2() #0 section ".ext_fun2_attr"
+// CHECK: define internal void @int_fun() #0 section ".int_fun_attr"
+// CHECK: define internal void @int_fun2() #0 section ".int_fun2_attr"
+// Function attributes should not include implicit-section-name.
+// CHECK-NOT: attributes #0 = {{.*}}implicit-section-name
+// No other attribute group should be present in the file.
+// CHECK-NOT: attributes #1
Index: lib/CodeGen/CodeGenModule.cpp
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -1485,10 +1485,6 @@
   setLinkageForGV(F, FD);
   setGVProperties(F, FD);
-  if (FD->getAttr<PragmaClangTextSectionAttr>()) {
-    F->addFnAttr("implicit-section-name");
-  }
   if (const SectionAttr *SA = FD->getAttr<SectionAttr>())

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