[gcc r11-8669] Change EH pointer encodings to PC relative on Windows

[gcc r11-8669] Change EH pointer encodings to PC relative on Windows

[Eric Botcazou]

https://gcc.gnu.org/g:014e6aa467b1648d09eab9897ca367bfec5e6b76

commit r11-8669-g014e6aa467b1648d09eab9897ca367bfec5e6b76
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Fri Jul 2 10:21:11 2021 +0200

    Change EH pointer encodings to PC relative on Windows
    
    A big difference between ELF and PE-COFF is that, with the latter, you can
    build position-independent executables or DLLs without generating PIC; as
    a  matter of fact, flag_pic has historically been forced to 0 for 32-bit:
    
    /* Don't allow flag_pic to propagate since gas may produce invalid code
       otherwise.  */
    
    \
    do {
            \
      flag_pic = TARGET_64BIT ? 1 : 0; \
    } while (0)
    
    The reason is that the linker builds a .reloc section that collects the
    absolute relocations in the generated binary, and the loader uses them to
    relocate it at load time if need be (e.g. if --dynamicbase is enabled).
    
    Up to binutils 2.35, the GNU linker didn't build the .reloc section for
    executables and defaulted to --enable-auto-image-base for DLLs, which means
    that DLLs had an essentially unique load address and, therefore, need not
    be relocated by the loader in most cases.
    
    With binutils 2.36 and later, the GNU linker builds a .reloc section for
    executables (thus making them PIE), --enable-auto-image-base is disabled
    and --dynamicbase is enabled by default, which means that essentially all
    the binaries are relocated at load time.
    
    This badly breaks the 32-bit compiler configured to use DWARF-2 EH because
    the loader corrupts the .eh_frame section when processing the relocations
    contained in the .reloc section.
    
    gcc/
            * config/i386/i386.c (asm_preferred_eh_data_format): Always use the
            PIC encodings for PE-COFF targets.

Diff:
---
 gcc/config/i386/i386.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 3bec713a0e8..36684c7857c 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -21761,10 +21761,12 @@ ix86_stack_protect_fail (void)
    After all, the relocation needed is the same as for the call insn.
    Whether or not a particular assembler allows us to enter such, I
    guess we'll have to see.  */
+
 int
 asm_preferred_eh_data_format (int code, int global)
 {
-  if (flag_pic)
+  /* PE-COFF is effectively always -fPIC because of the .reloc section.  */
+  if (flag_pic || TARGET_PECOFF)
     {
       int type = DW_EH_PE_sdata8;
       if (!TARGET_64BIT
@@ -21773,9 +21775,11 @@ asm_preferred_eh_data_format (int code, int global)
 	type = DW_EH_PE_sdata4;
       return (global ? DW_EH_PE_indirect : 0) | DW_EH_PE_pcrel | type;
     }
+
   if (ix86_cmodel == CM_SMALL
       || (ix86_cmodel == CM_MEDIUM && code))
     return DW_EH_PE_udata4;
+
   return DW_EH_PE_absptr;
 }