[libunwind] Don't assume the return address register is always saved and has CFI for it

[thegameg]

Summary

If we want to take shrink-wrapping to the next level, we will shrink-wrap the return address register, and realize that on some paths we don't actually need to save it.

In this case, libunwind fails to set IP to the return address, since there are no CFI directives on how to get its value in that IP range.

The following fix gets the return address from the current return address register, and if there are CFI directives for it, it will be overriden.

Not sure how libunwind is tested (it has only 4 tests in the test directory). Probably the test case can be reduced or dropped since it comes from the llvm test-suite.

[hfinkel]

src/DwarfInstructions.hpp

168–172

Please start sentences in comments with a capital letter.

[thegameg]

Won't be back to this anytime soon.

src/DwarfInstructions.hpp

if (CFI_Parser<A>::decodeFDE(addressSpace, fdeStart, &fdeInfo,
     PrologInfo prolog;
     if (CFI_Parser<A>::parseFDEInstructions(addressSpace, fdeInfo, cieInfo, pc,
                                             &prolog)) {
       // get pointer to cfa (architecture specific)
       pint_t cfa = getCFA(addressSpace, prolog, registers);
 
        // restore registers that DWARF says were saved
       R newRegisters = registers;
-      pint_t returnAddress = 0;
+      // Assume the return address is in the return address register, first,
+      // then if we find CFI for it, we get it from there. This allows
+      // shrink-wrapping to be possible for the return address register.
+      pint_t returnAddress =
+          registers.getRegister((int)cieInfo.returnAddressRegister);
       const int lastReg = R::lastDwarfRegNum();
       assert((int)CFI_Parser<A>::kMaxRegisterNumber > lastReg &&
              "register range too large");
       assert(lastReg >= (int)cieInfo.returnAddressRegister &&
              "register range does not contain return address register");
       for (int i = 0; i <= lastReg; ++i) {
         if (prolog.savedRegisters[i].location !=
             CFI_Parser<A>::kRegisterUnused) {

test/recursive-throw-shrink-wrap.s

+# This comes from SingleSource/Regression/C++/EH/Regression-C++-recursive-throw with advanced shrink-wrapping enabled.
+  .section  __TEXT,__text,regular,pure_instructions
+  .ios_version_min 5, 0
+  .globl  __Z3thri                ; -- Begin function _Z3thri
+  .p2align  2
+__Z3thri:                               ; @_Z3thri
+  .cfi_startproc
+; BB#0:                                 ; %entry
+  cmp   w0, #1          ; =1
+  b.le  LBB0_2
+; BB#1:                                 ; %if.end
+  stp  x29, x30, [sp, #-16]!   ; 8-byte Folded Spill
+Lcfi0:
+  .cfi_def_cfa w29, 16
+Lcfi1:
+  .cfi_offset w29, -16
+Lcfi2:
+  .cfi_offset w30, -8
+  mov  x29, sp
+  sub w0, w0, #2              ; =2
+  bl  __Z3thri
+  ldp  x29, x30, [sp], #16     ; 8-byte Folded Reload
+Lcfi3:
+  .cfi_restore w30
+Lcfi4:
+  .cfi_restore w29
+  ret
+LBB0_2:                                 ; %invoke.cont
+  adrp  x8, _thrown@PAGE
+  orr w9, wzr, #0x1
+  strb  w9, [x8, _thrown@PAGEOFF]
+  orr w0, wzr, #0x1
+  bl  ___cxa_allocate_exception
+Lloh0:
+  adrp  x1, __ZTI13TestException@PAGE
+Lloh1:
+  add x1, x1, __ZTI13TestException@PAGEOFF
+  mov x2, #0
+  bl  ___cxa_throw
+  .loh AdrpAdd  Lloh0, Lloh1
+  .cfi_endproc
+                                        ; -- End function
+  .private_extern ___clang_call_terminate ; -- Begin function __clang_call_terminate
+  .globl  ___clang_call_terminate
+  .weak_def_can_be_hidden ___clang_call_terminate
+  .p2align  2
+___clang_call_terminate:                ; @__clang_call_terminate
+; BB#0:
+  stp  x29, x30, [sp, #-16]!   ; 8-byte Folded Spill
+  bl  ___cxa_begin_catch
+  bl  __ZSt9terminatev
+                                        ; -- End function
+  .globl  __Z3runv                ; -- Begin function _Z3runv
+  .p2align  2
+__Z3runv:                               ; @_Z3runv
+Lfunc_begin0:
+  .cfi_startproc
+  .cfi_personality 155, ___gxx_personality_v0
+  .cfi_lsda 16, Lexception0
+; BB#0:                                 ; %entry
+  stp  x29, x30, [sp, #-16]!   ; 8-byte Folded Spill
+Lcfi5:
+  .cfi_def_cfa w29, 16
+Lcfi6:
+  .cfi_offset w29, -16
+Lcfi7:
+  .cfi_offset w30, -8
+  mov  x29, sp
+Ltmp0:
+  orr w0, wzr, #0x2
+  bl  __Z3thri
+Ltmp1:
+; BB#1:                                 ; %invoke.cont
+Ltmp2:
+  bl  _abort
+Ltmp3:
+; BB#2:                                 ; %invoke.cont1
+LBB2_3:                                 ; %lpad
+Ltmp4:
+  bl  ___cxa_begin_catch
+  adrp  x8, _caught@PAGE
+  orr w9, wzr, #0x1
+  strb  w9, [x8, _caught@PAGEOFF]
+  ldp  x29, x30, [sp], #16     ; 8-byte Folded Reload
+Lcfi8:
+  .cfi_restore w30
+Lcfi9:
+  .cfi_restore w29
+  b ___cxa_end_catch
+Lfunc_end0:
+  .cfi_endproc
+  .section  __TEXT,__gcc_except_tab
+  .p2align  2
+GCC_except_table2:
+Lexception0:
+  .byte 255                     ; @LPStart Encoding = omit
+  .byte 155                     ; @TType Encoding = indirect pcrel sdata4
+  .asciz  "\242\200\200"          ; @TType base offset
+  .byte 3                       ; Call site Encoding = udata4
+  .byte 26                      ; Call site table length
+Lset0 = Ltmp0-Lfunc_begin0              ; >> Call Site 1 <<
+  .long Lset0
+Lset1 = Ltmp3-Ltmp0                     ;   Call between Ltmp0 and Ltmp3
+  .long Lset1
+Lset2 = Ltmp4-Lfunc_begin0              ;     jumps to Ltmp4
+  .long Lset2
+  .byte 1                       ;   On action: 1
+Lset3 = Ltmp3-Lfunc_begin0              ; >> Call Site 2 <<
+  .long Lset3
+Lset4 = Lfunc_end0-Ltmp3                ;   Call between Ltmp3 and Lfunc_end0
+  .long Lset4
+  .long 0                       ;     has no landing pad
+  .byte 0                       ;   On action: cleanup
+  .byte 1                       ; >> Action Record 1 <<
+                                        ;   Catch TypeInfo 1
+  .byte 0                       ;   No further actions
+                                        ; >> Catch TypeInfos <<
+Ltmp5:                                  ; TypeInfo 1
+  .long __ZTI13TestException@GOT-Ltmp5
+  .p2align  2
+                                        ; -- End function
+  .section  __TEXT,__text,regular,pure_instructions
+  .globl  _main                   ; -- Begin function main
+  .p2align  2
+_main:                                  ; @main
+Lfunc_begin1:
+  .cfi_startproc
+  .cfi_personality 155, ___gxx_personality_v0
+  .cfi_lsda 16, Lexception1
+; BB#0:                                 ; %entry
+  stp  x29, x30, [sp, #-16]!   ; 8-byte Folded Spill
+Lcfi10:
+  .cfi_def_cfa w29, 16
+Lcfi11:
+  .cfi_offset w29, -16
+Lcfi12:
+  .cfi_offset w30, -8
+  mov  x29, sp
+Ltmp6:
+  bl  __Z3runv
+Ltmp7:
+; BB#1:                                 ; %try.cont
+  adrp  x8, _thrown@PAGE
+  ldrb  w8, [x8, _thrown@PAGEOFF]
+  adrp  x9, _caught@PAGE
+  ldrb  w9, [x9, _caught@PAGEOFF]
+  cmp   w9, #0          ; =0
+  cset   w9, eq
+  eor w8, w8, #0x1
+  and w8, w8, #0xff
+  orr   w0, w9, w8
+  ldp  x29, x30, [sp], #16     ; 8-byte Folded Reload
+Lcfi13:
+  .cfi_restore w30
+Lcfi14:
+  .cfi_restore w29
+  ret
+LBB3_2:                                 ; %lpad
+Lcfi15:
+  .cfi_offset w30, -8
+Lcfi16:
+  .cfi_offset w29, -16
+Ltmp8:
+  bl  ___cxa_begin_catch
+Ltmp9:
+  bl  _abort
+Ltmp10:
+; BB#3:                                 ; %invoke.cont2
+LBB3_4:                                 ; %lpad1
+Ltmp11:
+  mov  x19, x0
+Ltmp12:
+  bl  ___cxa_end_catch
+Ltmp13:
+; BB#5:                                 ; %eh.resume
+  mov  x0, x19
+  bl  __Unwind_Resume
+LBB3_6:                                 ; %terminate.lpad
+Ltmp14:
+  bl  ___clang_call_terminate
+Lfunc_end1:
+  .cfi_endproc
+  .section  __TEXT,__gcc_except_tab
+  .p2align  2
+GCC_except_table3:
+Lexception1:
+  .byte 255                     ; @LPStart Encoding = omit
+  .byte 155                     ; @TType Encoding = indirect pcrel sdata4
+  .byte 73                      ; @TType base offset
+  .byte 3                       ; Call site Encoding = udata4
+  .byte 65                      ; Call site table length
+Lset5 = Ltmp6-Lfunc_begin1              ; >> Call Site 1 <<
+  .long Lset5
+Lset6 = Ltmp7-Ltmp6                     ;   Call between Ltmp6 and Ltmp7
+  .long Lset6
+Lset7 = Ltmp8-Lfunc_begin1              ;     jumps to Ltmp8
+  .long Lset7
+  .byte 1                       ;   On action: 1
+Lset8 = Ltmp7-Lfunc_begin1              ; >> Call Site 2 <<
+  .long Lset8
+Lset9 = Ltmp9-Ltmp7                     ;   Call between Ltmp7 and Ltmp9
+  .long Lset9
+  .long 0                       ;     has no landing pad
+  .byte 0                       ;   On action: cleanup
+Lset10 = Ltmp9-Lfunc_begin1             ; >> Call Site 3 <<
+  .long Lset10
+Lset11 = Ltmp10-Ltmp9                   ;   Call between Ltmp9 and Ltmp10
+  .long Lset11
+Lset12 = Ltmp11-Lfunc_begin1            ;     jumps to Ltmp11
+  .long Lset12
+  .byte 0                       ;   On action: cleanup
+Lset13 = Ltmp12-Lfunc_begin1            ; >> Call Site 4 <<
+  .long Lset13
+Lset14 = Ltmp13-Ltmp12                  ;   Call between Ltmp12 and Ltmp13
+  .long Lset14
+Lset15 = Ltmp14-Lfunc_begin1            ;     jumps to Ltmp14
+  .long Lset15
+  .byte 1                       ;   On action: 1
+Lset16 = Ltmp13-Lfunc_begin1            ; >> Call Site 5 <<
+  .long Lset16
+Lset17 = Lfunc_end1-Ltmp13              ;   Call between Ltmp13 and Lfunc_end1
+  .long Lset17
+  .long 0                       ;     has no landing pad
+  .byte 0                       ;   On action: cleanup
+  .byte 1                       ; >> Action Record 1 <<
+                                        ;   Catch TypeInfo 1
+  .byte 0                       ;   No further actions
+                                        ; >> Catch TypeInfos <<
+  .long 0                       ; TypeInfo 1
+  .p2align  2
+                                        ; -- End function
+  .globl  _thrown                 ; @thrown
+.zerofill __DATA,__common,_thrown,1,0
+  .globl  _caught                 ; @caught
+.zerofill __DATA,__common,_caught,1,0
+  .private_extern __ZTS13TestException ; @_ZTS13TestException
+  .section  __TEXT,__const
+  .globl  __ZTS13TestException
+  .weak_definition  __ZTS13TestException
+__ZTS13TestException:
+  .asciz  "13TestException"
+
+  .private_extern __ZTI13TestException ; @_ZTI13TestException
+  .section  __DATA,__data
+  .globl  __ZTI13TestException
+  .weak_definition  __ZTI13TestException
+  .p2align  3
+__ZTI13TestException:
+  .quad __ZTVN10__cxxabiv117__class_type_infoE+16
+  .quad __ZTS13TestException-9223372036854775808
+
+
+.subsections_via_symbols