[patch] Fix PR target/81361

[Eric Botcazou]

Hi,

exception handling is currently broken in all languages for Darwin at -O2 on 
the mainline because of what appears to be a bug in either the assembler or 
the system unwinder.  The problem occurs when the compiler decides to split a 
function into hot & cold parts and the cold part is active wrt EH; in this 
case, the compiler generates a FDE for each part (the Darwin port doesn't use 
the CFI assembler directives) and the second FDE looks like:

.set L$set$24,LEFDE3-LASFDE3
        .long L$set$24  # FDE Length
LASFDE3:
        .long   LASFDE3-EH_frame1       # FDE CIE offset
        .quad   LCOLDB1-.       # FDE initial location
        .set L$set$25,LCOLDE1-LCOLDB1
        .quad L$set$25  # FDE address range
        .byte   0x8     # uleb128 0x8; Augmentation size
        .quad   LLSDAC5-.       # Language Specific Data Area
        .byte   0x1     # DW_CFA_set_loc
        .quad   LCFI1-.
        .byte   0xe     # DW_CFA_def_cfa_offset
        .byte   0x10    # uleb128 0x10
        .byte   0x83    # DW_CFA_offset, column 0x3
        .byte   0x2     # uleb128 0x2

Note the DW_CFA_set_loc operation: it's the only case where the compiler emits 
it (DW_CFA_advance_loc4 is usually emitted) and is the source of the problem, 
since it appears that the PC-relative relocation is not applied to the operand 
of the DW_CFA_set_loc (unlike to the 2 other cases in the FDE).

This DW_CFA_set_loc instruction is emitted by add_cfis_to_fde for the second 
FDE generated for the cold part of a function but doesn't seem necessary any 
more, since there is a label (LCOLDB1) to be used now (this can also be seen 
on Linux with the -fno-dwarf2-cfi-asm option).

Bootstrapped/regtested on x86-64/Linux by me and various versions of Darwin by 
Iain, Dominique and myself.  OK for the mainline?


2017-09-18  Eric Botcazou  <ebotca...@adacore.com>

        PR target/81361
        * dwarf2cfi.c (add_cfis_to_fde): Do not generate DW_CFA_set_loc
        after switching to a new text section.

-- 
Eric Botcazou
Index: dwarf2cfi.c
===================================================================
--- dwarf2cfi.c	(revision 252749)
+++ dwarf2cfi.c	(working copy)
@@ -2209,20 +2209,13 @@ add_cfis_to_fde (void)
 {
   dw_fde_ref fde = cfun->fde;
   rtx_insn *insn, *next;
-  /* We always start with a function_begin label.  */
-  bool first = false;
 
   for (insn = get_insns (); insn; insn = next)
     {
       next = NEXT_INSN (insn);
 
       if (NOTE_P (insn) && NOTE_KIND (insn) == NOTE_INSN_SWITCH_TEXT_SECTIONS)
-	{
-	  fde->dw_fde_switch_cfi_index = vec_safe_length (fde->dw_fde_cfi);
-	  /* Don't attempt to advance_loc4 between labels
-	     in different sections.  */
-	  first = true;
-	}
+	fde->dw_fde_switch_cfi_index = vec_safe_length (fde->dw_fde_cfi);
 
       if (NOTE_P (insn) && NOTE_KIND (insn) == NOTE_INSN_CFI)
 	{
@@ -2247,8 +2240,7 @@ add_cfis_to_fde (void)
 
 	      /* Set the location counter to the new label.  */
 	      xcfi = new_cfi ();
-	      xcfi->dw_cfi_opc = (first ? DW_CFA_set_loc
-				  : DW_CFA_advance_loc4);
+	      xcfi->dw_cfi_opc = DW_CFA_advance_loc4;
 	      xcfi->dw_cfi_oprnd1.dw_cfi_addr = label;
 	      vec_safe_push (fde->dw_fde_cfi, xcfi);
 
@@ -2263,7 +2255,6 @@ add_cfis_to_fde (void)
 	      insn = NEXT_INSN (insn);
 	    }
 	  while (insn != next);
-	  first = false;
 	}
     }
 }