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gc sections and .eh_frame

Alan Modra amodra@bigpond.net.au
Thu Jun 9 12:49:00 GMT 2005 
On Thu, Jun 09, 2005 at 01:07:58PM +0100, Jonathan Larmour wrote:
> Can I ask where the used .gcc_except_table.* sections _are_ meant to be 
> marked?  I cannot see anywhere that _bfd_elf_discard_section_eh_frame does 
> it, for example. Why would .gcc_except_table.* get marked, but .rodata not 
> get marked for powerpc?

I think Eric cheated, and put a KEEP in the linker script.

As far as .eh_frame is concerned, the problem is that gcc doesn't emit
eh info to uniquely named sections for each function when compiling with
-ffunction-sections -fdata-sections.  Instead, you get it all lumped
into one section.  Since ld's --gc-sections option works by looking at
relocs on a section by section basis, if .eh_frame is kept, then all
sections referred to by .eh_frame will be kept.  And .eh_frame typically
references function code sections..

Relocation section '.rela.eh_frame' at offset 0x1293c contains 15 entries:
 Offset     Info    Type                Sym. Value  Symbol's Name + Addend
00000012  0000b901 R_PPC_ADDR32           00000000   __gxx_personality_v0 + 0
00000024  00000801 R_PPC_ADDR32           00000000   .text._ZNSt8ios_base17_M_call_callbacksENS_5eventE + 0
0000002d  00000901 R_PPC_ADDR32           00000000   .rodata + 0
00000048  00000a01 R_PPC_ADDR32           00000000   .text._ZNSt8ios_base20_M_dispose_callbacksEv + 0
00000068  00000b01 R_PPC_ADDR32           00000000   .text._ZNSt8ios_baseD2Ev + 0
00000071  00000901 R_PPC_ADDR32           00000000   .rodata + 1d
00000088  00000c01 R_PPC_ADDR32           00000000   .text._ZNSt8ios_baseD1Ev + 0
00000091  00000901 R_PPC_ADDR32           00000000   .rodata + 2a
000000a8  00000d01 R_PPC_ADDR32           00000000   .text._ZNSt8ios_baseD0Ev + 0
000000b1  00000901 R_PPC_ADDR32           00000000   .rodata + 37
000000c8  00001001 R_PPC_ADDR32           00000000   .text._ZNSt8ios_base6xallocEv + 0
000000d1  00000901 R_PPC_ADDR32           00000000   .rodata + 48
000000e4  00001101 R_PPC_ADDR32           00000000   .text._ZNSt8ios_base17register_callbackEPFvNS_5eventERS_iEi + 0
0000010c  00001501 R_PPC_ADDR32           00000000   .text._ZNSt8ios_base13_M_grow_wordsEib + 0
00000115  00000901 R_PPC_ADDR32           00000000   .rodata + 5c


-- 
Alan Modra
IBM OzLabs - Linux Technology Centre
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