[BOLT] Fix ignored LP at fragment start

[FPar]

Summary

If the first block of a fragment is also a landing pad, the landing pad
is not used if an exception is thrown. This is because the landing pad
is at the same start address that the corresponding LSDA describes. In
that case, the offset in the call site records to refer to that landing
pad is zero, and a zero offset is interpreted by the personality
function as "no handler" and ignored.

[FPar]

Replace zero with trap instruction

[Amir]

bolt/test/runtime/X86/Inputs/lp-fragment-start.fdata

1 ↗

(On Diff #453435)

You can merge fdata into the assembly source by leveraging link_fdata script. See e.g. bolt/test/X86/lto-name-match.s

[FPar]

Inline fdata into assembly

[FPar]

bolt/test/runtime/X86/Inputs/lp-fragment-start.fdata

1 ↗

(On Diff #453435)

I didn't know that, thanks!

[FPar]

Use symbol to calculate offset for fdata

[Amir]

Thanks for fixing this corner case!

bolt/lib/Core/BinaryEmitter.cpp

 void BinaryEmitter::emitFunctionBody(BinaryFunction &BF, bool EmitColdPart,
                                      bool EmitCodeOnly) {
   if (!EmitCodeOnly && EmitColdPart && BF.hasConstantIsland())
     BF.duplicateConstantIslands();
 
   const FunctionFragment FF = BF.getLayout().getFragment(
       EmitColdPart ? FragmentNum::cold() : FragmentNum::hot());
 
+  if (!FF.empty() && FF.front()->isLandingPad()) {
+    assert(!FF.front()->isEntryPoint() &&
+           "Landing pad cannot be entry point of function");
+    // If the first block of the fragment is a landing pad, it's offset from the
+    // start of the area that the corresponding LSDA describes is zero. In this
+    // case, the call site entries in that LSDA have 0 as offset to the landing
+    // pad, which the runtime interprets as "no handler". To prevent this,
+    // insert some padding.
+    Streamer.emitIntValue(BC.MIB->getTrapFillValue(), 1);
+  }
+
   // Track the first emitted instruction with debug info.
   bool FirstInstr = true;
   for (BinaryBasicBlock *const BB : FF) {
     if ((opts::AlignBlocks || opts::PreserveBlocksAlignment) &&
         BB->getAlignment() > 1)
       Streamer.emitCodeAlignment(BB->getAlignment(), &*BC.STI,
                                  BB->getAlignmentMaxBytes());
     Streamer.emitLabel(BB->getLabel());

bolt/test/runtime/X86/lp-fragment-start.s

+# Test that landing pads at fragment entries are not omitted during unwinding.
+# This uses profile2 splitting with a fake fdata, so SplitFunctions splits main
+# right before landing pad.
+
+# RUN: llvm-mc --filetype=obj --triple x86_64-unknown-unknown %s -o %t.o
+# RUN: link_fdata %s %t.o %t.fdata
+# RUN: llvm-strip --strip-unneeded %t.o
+# RUN: %clang++ %cflags %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt --split-functions --data=%t.fdata \
+# RUN:         --print-split --print-only=main --split-eh --split-all-cold \
+# RUN:     2>&1 | FileCheck --check-prefix=BOLT-CHECK %s
+# RUN: %t.bolt | FileCheck --check-prefix=RUN-CHECK %s
+
+# BOLT-CHECK: -------   HOT-COLD SPLIT POINT   -------
+# BOLT-CHECK-EMPTY:
+# BOLT-CHECK-NEXT: .LLP1
+
+# RUN-CHECK: failed successfully
+
+
+        .text
+        .file   "lp-fragment-start.cpp"
+        .globl  main                            # -- Begin function main
+        .p2align        4, 0x90
+        .type   main,@function
+main:                                   # @main
+.Lfunc_begin0:
+        .cfi_startproc
+        .cfi_personality 155, DW.ref.__gxx_personality_v0
+        .cfi_lsda 27, .Lexception0
+        pushq   %r15
+        .cfi_def_cfa_offset 16
+        pushq   %r14
+        .cfi_def_cfa_offset 24
+        pushq   %rbx
+        .cfi_def_cfa_offset 32
+        .cfi_offset %rbx, -32
+        .cfi_offset %r14, -24
+        .cfi_offset %r15, -16
+        movl    $16, %edi
+LL_fdata0:
+        callq   __cxa_allocate_exception@PLT
+# FDATA: 1 main #LL_fdata0# 1 __cxa_allocate_exception@PLT 0 0 1
+        movq    %rax, %rbx
+.Ltmp0:
+        leaq    .L.str(%rip), %rsi
+        movq    %rax, %rdi
+        callq   _ZNSt13runtime_errorC1EPKc@PLT
+        jmp     .Ltmp3
+.Ltmp1:
+# Cause split here, so .Ltmp5 is the first block of the fragment
+.Ltmp5: # LP for .Ltmp3 to .Ltmp4
+        movq    %rdx, %r15
+        movq    %rax, %r14
+        jmp     .LBB0_4
+.Ltmp3: # throw std::runtime_error
+        movq    _ZTISt13runtime_error@GOTPCREL(%rip), %rsi
+        movq    _ZNSt13runtime_errorD1Ev@GOTPCREL(%rip), %rdx
+        movq    %rbx, %rdi
+        callq   __cxa_throw@PLT # LP: .Ltmp5
+.Ltmp4:
+.Ltmp2:
+        movq    %rdx, %r15
+        movq    %rax, %r14
+        movq    %rbx, %rdi
+        callq   __cxa_free_exception@PLT
+.LBB0_4:
+        movq    %r14, %rdi
+        cmpl    $1, %r15d
+        jne     .LBB0_5
+        callq   __cxa_begin_catch@PLT
+        movq    (%rax), %rcx
+        movq    %rax, %rdi
+        callq   *16(%rcx)
+        movq    %rax, %rdi
+        callq   puts@PLT
+        callq   __cxa_end_catch@PLT
+        xorl    %eax, %eax
+        popq    %rbx
+        .cfi_def_cfa_offset 24
+        popq    %r14
+        .cfi_def_cfa_offset 16
+        popq    %r15
+        .cfi_def_cfa_offset 8
+        retq
+.LBB0_5:
+        .cfi_def_cfa_offset 32
+        callq   _Unwind_Resume@PLT
+.Lfunc_end0:
+        .size   main, .Lfunc_end0-main
+        .cfi_endproc
+        .section        .gcc_except_table,"a",@progbits
+        .p2align        2
+GCC_except_table0:
+.Lexception0:
+        .byte   255                             # @LPStart Encoding = omit
+        .byte   155                             # @TType Encoding = indirect pcrel sdata4
+        .uleb128 .Lttbase0-.Lttbaseref0
+.Lttbaseref0:
+        .byte   1                               # Call site Encoding = uleb128
+        .uleb128 .Lcst_end0-.Lcst_begin0
+.Lcst_begin0:
+        .uleb128 .Lfunc_begin0-.Lfunc_begin0    # >> Call Site 1 <<
+        .uleb128 .Ltmp0-.Lfunc_begin0           #   Call between .Lfunc_begin0 and .Ltmp0
+        .byte   0                               #     has no landing pad
+        .byte   0                               #   On action: cleanup
+        .uleb128 .Ltmp0-.Lfunc_begin0           # >> Call Site 2 <<
+        .uleb128 .Ltmp1-.Ltmp0                  #   Call between .Ltmp0 and .Ltmp1
+        .uleb128 .Ltmp2-.Lfunc_begin0           #     jumps to .Ltmp2
+        .byte   3                               #   On action: 2
+        .uleb128 .Ltmp3-.Lfunc_begin0           # >> Call Site 3 <<
+        .uleb128 .Ltmp4-.Ltmp3                  #   Call between .Ltmp3 and .Ltmp4
+        .uleb128 .Ltmp5-.Lfunc_begin0           #     jumps to .Ltmp5
+        .byte   5                               #   On action: 3
+        .uleb128 .Ltmp4-.Lfunc_begin0           # >> Call Site 4 <<
+        .uleb128 .Lfunc_end0-.Ltmp4             #   Call between .Ltmp4 and .Lfunc_end0
+        .byte   0                               #     has no landing pad
+        .byte   0                               #   On action: cleanup
+.Lcst_end0:
+        .byte   0                               # >> Action Record 1 <<
+        .byte   0                               #   No further actions
+        .byte   1                               # >> Action Record 2 <<
+        .byte   125                             #   Continue to action 1
+        .byte   1                               # >> Action Record 3 <<
+        .byte   0                               #   No further actions
+        .p2align        2
+.Ltmp6:                                 # TypeInfo 1
+        .long   .L_ZTISt13runtime_error.DW.stub-.Ltmp6
+.Lttbase0:
+        .p2align        2
+        .type   .L.str,@object                  # @.str
+        .section        .rodata.str1.1,"aMS",@progbits,1
+.L.str:
+        .asciz  "failed successfully"
+        .size   .L.str, 20
+
+        .data
+        .p2align        3
+.L_ZTISt13runtime_error.DW.stub:
+        .quad   _ZTISt13runtime_error
+        .hidden DW.ref.__gxx_personality_v0
+        .weak   DW.ref.__gxx_personality_v0
+        .section        .data.DW.ref.__gxx_personality_v0,"aGw",@progbits,DW.ref.__gxx_personality_v0,comdat
+        .p2align        3
+        .type   DW.ref.__gxx_personality_v0,@object
+        .size   DW.ref.__gxx_personality_v0, 8
+DW.ref.__gxx_personality_v0:
+        .quad   __gxx_personality_v0
+        .ident  "clang version 14.0.0 (https://github.com/llvm/llvm-project.git 329fda39c507e8740978d10458451dcdb21563be)"
+        .section        ".note.GNU-stack","",@progbits
+        .addrsig
+        .addrsig_sym __gxx_personality_v0
+        .addrsig_sym _Unwind_Resume
+        .addrsig_sym _ZTISt13runtime_error
+
+
+# #include <cstdio>
+# #include <stdexcept>
+#
+# int main() {
+#     try {
+#         throw std::runtime_error("failed successfully");
+#     } catch (const std::runtime_error &e) {
+#         puts(e.what());
+#     }
+#     return 0;
+# }