| Message ID | alpine.LFD.2.21.2011301023470.656242@eddie.linux-mips.org |
|---|---|
| State | Accepted |
| Headers | show |
| Series | VAX: Fix the LTO compiler downgrading code to non-PIC model | expand |
On 11/30/20 8:11 AM, Maciej W. Rozycki wrote: > Fix a testsuite failure: > > /tmp/ccL65Mmt.s: Assembler messages: > /tmp/ccL65Mmt.s:36: Warning: Symbol n used as immediate operand in PIC mode. > FAIL: gcc.dg/lto/pr55660 c_lto_pr55660_0.o-c_lto_pr55660_1.o link, -O0 -flto -flto-partition=none -fuse-linker-plugin > > where non-PIC code is substituted by the LTO compiler at the link stage > for what used to be PIC code in the original compilation. This happens > because in the de-facto VAX ELF psABI we rely on code being PIC for GOT > support in dynamic executables and arrange that by having `-fPIC' passed > to the compiler by default by means of a specs recipe. > > That is however canceled where the LTO wrapper is used, by an internal > arrangement in the LTO compiler that clears the PIC flag whenever the > `-flinker-output=exec' option has been used. This has been deliberately > introduced with commit 1ff9ed6fb282 ("re PR lto/67548 (LTO drops weak > binding with "ld -r")")[1]: > > "In the log of PR67548 HJ actually pointed out that we do have API at > linker plugin side which says what type of output is done. This is cool > because we can also use it to drop -fpic when building static binary. > This is common in Firefox, where some objects are built with -fpic and > linked to both binaries and libraries." > > with this code: > > case LTO_LINKER_OUTPUT_EXEC: /* Normal executable */ > flag_pic = 0; > flag_pie = 0; > flag_shlib = 0; > break; > > Consequently code like: > > .L6: > addl3 -8(%fp),$n,%r0 > pushl %r0 > calls $1,foo > addl2 %r0,-12(%fp) > incl -8(%fp) > .L5: > > is produced by the LTO compiler, where a reference to `n' is used that > is invalid in PIC code, because it uses the immediate addressing mode, > denoted by the `$' prefix. > > For that not to happen we must never pass `-flinker-output=exec' to the > LTO compiler unless non-PIC code has been explicitly requested. Using > `-flinker-output=dyn' except for relocatable output would seem the > simplest approach, as it does not fiddle with any of the internal code > model settings beyond what the command-line options have arranged and > therefore lets them remain the same as with the original compilation, > but it breaks as well causing PR lto/69866 to retrigger, as that code > seems sensitive to `flag_shlib': > > lto1: internal compiler error: in add_symbol_to_partition_1, at lto/lto-partition.c:152 > 0x105be1cb add_symbol_to_partition_1 > .../gcc/lto/lto-partition.c:152 > 0x105be443 add_symbol_to_partition_1 > .../gcc/lto/lto-partition.c:194 > 0x105be80f add_symbol_to_partition > .../gcc/lto/lto-partition.c:270 > 0x105bee6f add_sorted_nodes > .../gcc/lto/lto-partition.c:395 > 0x105c0903 lto_balanced_map(int, int) > .../gcc/lto/lto-partition.c:815 > 0x105aa91f do_whole_program_analysis > .../gcc/lto/lto.c:499 > 0x105aac97 lto_main() > .../gcc/lto/lto.c:637 > Please submit a full bug report, > with preprocessed source if appropriate. > Please include the complete backtrace with any bug report. > See <https://gcc.gnu.org/bugs/> for instructions. > lto-wrapper: fatal error: .../gcc/xgcc returned 1 exit status > compilation terminated. > .../usr/bin/vax-netbsdelf-ld: error: lto-wrapper failed > collect2: error: ld returned 1 exit status > compiler exited with status 1 > FAIL: gcc.dg/lto/pr69866 c_lto_pr69866_0.o-c_lto_pr69866_1.o link, -O0 -flto -fuse-linker-plugin -fno-fat-lto-objects (internal compiler error) > > Substitute `-flinker-output=pie' for `-flinker-output=exec' in the specs > then unless `-no-pie' has also been used, preserving the original intent > of emitting PIC code by default for executables while keeping the linker > arrangement unchanged. The LTO compiler uses the `cc1' spec, so keep > `cc1plus' unmodified. > > This makes code like: > > .L6: > movab n,%r0 > addl2 -8(%fp),%r0 > pushl %r0 > calls $1,foo > addl2 %r0,-12(%fp) > incl -8(%fp) > .L5: > > be produced instead corresponding to the fragment quoted above, which is > valid PIC code as it uses the PC-relative addressing mode denoted by the > absence of a prefix to `n' (which can be redirected to GOT as required, > by changing the addressing mode to PC-relative indirect in the operand > specifier). > > Ideally we would instead default to the PIE model for executables, but > that triggers a BFD bug where for a change the LTO wrapper is not used: > > .../usr/bin/vax-netbsdelf-ld: /tmp/ccV2sWQt.ltrans0.ltrans.o: warning: GOT addend of 3 to `n' does not match previous GOT addend of 0 > FAIL: gcc.dg/lto/pr55660 c_lto_pr55660_0.o-c_lto_pr55660_1.o link, -O2 -flto -flto-partition=1to1 -fno-use-linker-plugin > > which is due to assembly code like: > > main: > .word 0 > subl2 $4,%sp > movab n,%r0 > movab n+3,%r2 > clrl %r3 > movb $98,%r1 > .L4: > > and consequently object code like: > > 00000000 <main>: > 0: 00 00 .word 0x0000 # Entry mask: < > > 2: c2 04 5e subl2 $0x4,sp > 5: 9e ef 00 00 movab b <main+0xb>,r0 > 9: 00 00 50 > 7: R_VAX_GOT32 n > c: 9e ef 00 00 movab 12 <main+0x12>,r2 > 10: 00 00 52 > e: R_VAX_GOT32 n+0x3 > 13: d4 53 clrf r3 > 15: 90 8f 62 51 movb $0x62,r1 > > being produced. This would be problematic for external `n', because we > do not support multiple GOT entries for the same symbol referred to with > different offsets in a single link unit. In this case however the LTO > compiler correctly observes that `n' is defined by the executable and > not preemptible and therefore no GOT entry will be made for it. > > Indeed a valid executable is produced: > > 00010548 <main>: > 10548: 00 00 .word 0x0000 # Entry mask: < > > 1054a: c2 04 5e subl2 $0x4,sp > 1054d: 9e ef dd 14 movab 11a30 <n>,r0 > 10551: 00 00 50 > 10554: 9e ef d9 14 movab 11a33 <__bss_start>,r2 > 10558: 00 00 52 > 1055b: d4 53 clrf r3 > 1055d: 90 8f 62 51 movb $0x62,r1 > > despite the warning, but it would be rather bad to have users annoyed > with this message from BFD, however harmless, especially as it triggers > outside LTO compilations as well. > > Therefore this change is the best we can do until binutils have been > fixed. > > References: > > [1] Jan Hubicka, "Getting LTO incremental linking work", > <https://gcc.gnu.org/ml/gcc-patches/2015-11/msg02986.html> > > gcc/ > * config/vax/elf.h (VAX_CC1_SPEC, VAX_CC1PLUS_SPEC): New macros. > * config/vax/netbsd-elf.h (CC1_SPEC): Use VAX_CC1_SPEC rather > than VAX_CC1_AND_CC1PLUS_SPEC. > (CC1PLUS_SPEC): Use VAX_CC1PLUS_SPEC rather than > VAX_CC1_AND_CC1PLUS_SPEC. OK jeff
Index: gcc/gcc/config/vax/elf.h =================================================================== --- gcc.orig/gcc/config/vax/elf.h +++ gcc/gcc/config/vax/elf.h @@ -89,6 +89,16 @@ along with GCC; see the file COPYING3. %{!fpic: \ %{!fPIC:-fPIC}}}" +/* Don't let the LTO compiler switch the PIC options off. */ +#define VAX_CC1_SPEC \ + VAX_CC1_AND_CC1PLUS_SPEC \ + " %{flinker-output=exec" \ + ":%{no-pie:-flinker-output=exec;:-flinker-output=pie};" \ + ":%{flinker-output=*}}" \ + "%<flinker-output*" +#define VAX_CC1PLUS_SPEC \ + VAX_CC1_AND_CC1PLUS_SPEC + /* VAX ELF is always gas; override the generic VAX ASM_SPEC. */ #undef ASM_SPEC Index: gcc/gcc/config/vax/netbsd-elf.h =================================================================== --- gcc.orig/gcc/config/vax/netbsd-elf.h +++ gcc/gcc/config/vax/netbsd-elf.h @@ -35,10 +35,10 @@ along with GCC; see the file COPYING3. #endif #undef CC1_SPEC -#define CC1_SPEC NETBSD_CC1_AND_CC1PLUS_SPEC VAX_CC1_AND_CC1PLUS_SPEC +#define CC1_SPEC NETBSD_CC1_AND_CC1PLUS_SPEC VAX_CC1_SPEC #undef CC1PLUS_SPEC -#define CC1PLUS_SPEC NETBSD_CC1_AND_CC1PLUS_SPEC VAX_CC1_AND_CC1PLUS_SPEC +#define CC1PLUS_SPEC NETBSD_CC1_AND_CC1PLUS_SPEC VAX_CC1PLUS_SPEC #define NETBSD_ENTRY_POINT "__start"
Fix a testsuite failure: /tmp/ccL65Mmt.s: Assembler messages: /tmp/ccL65Mmt.s:36: Warning: Symbol n used as immediate operand in PIC mode. FAIL: gcc.dg/lto/pr55660 c_lto_pr55660_0.o-c_lto_pr55660_1.o link, -O0 -flto -flto-partition=none -fuse-linker-plugin where non-PIC code is substituted by the LTO compiler at the link stage for what used to be PIC code in the original compilation. This happens because in the de-facto VAX ELF psABI we rely on code being PIC for GOT support in dynamic executables and arrange that by having `-fPIC' passed to the compiler by default by means of a specs recipe. That is however canceled where the LTO wrapper is used, by an internal arrangement in the LTO compiler that clears the PIC flag whenever the `-flinker-output=exec' option has been used. This has been deliberately introduced with commit 1ff9ed6fb282 ("re PR lto/67548 (LTO drops weak binding with "ld -r")")[1]: "In the log of PR67548 HJ actually pointed out that we do have API at linker plugin side which says what type of output is done. This is cool because we can also use it to drop -fpic when building static binary. This is common in Firefox, where some objects are built with -fpic and linked to both binaries and libraries." with this code: case LTO_LINKER_OUTPUT_EXEC: /* Normal executable */ flag_pic = 0; flag_pie = 0; flag_shlib = 0; break; Consequently code like: .L6: addl3 -8(%fp),$n,%r0 pushl %r0 calls $1,foo addl2 %r0,-12(%fp) incl -8(%fp) .L5: is produced by the LTO compiler, where a reference to `n' is used that is invalid in PIC code, because it uses the immediate addressing mode, denoted by the `$' prefix. For that not to happen we must never pass `-flinker-output=exec' to the LTO compiler unless non-PIC code has been explicitly requested. Using `-flinker-output=dyn' except for relocatable output would seem the simplest approach, as it does not fiddle with any of the internal code model settings beyond what the command-line options have arranged and therefore lets them remain the same as with the original compilation, but it breaks as well causing PR lto/69866 to retrigger, as that code seems sensitive to `flag_shlib': lto1: internal compiler error: in add_symbol_to_partition_1, at lto/lto-partition.c:152 0x105be1cb add_symbol_to_partition_1 .../gcc/lto/lto-partition.c:152 0x105be443 add_symbol_to_partition_1 .../gcc/lto/lto-partition.c:194 0x105be80f add_symbol_to_partition .../gcc/lto/lto-partition.c:270 0x105bee6f add_sorted_nodes .../gcc/lto/lto-partition.c:395 0x105c0903 lto_balanced_map(int, int) .../gcc/lto/lto-partition.c:815 0x105aa91f do_whole_program_analysis .../gcc/lto/lto.c:499 0x105aac97 lto_main() .../gcc/lto/lto.c:637 Please submit a full bug report, with preprocessed source if appropriate. Please include the complete backtrace with any bug report. See <https://gcc.gnu.org/bugs/> for instructions. lto-wrapper: fatal error: .../gcc/xgcc returned 1 exit status compilation terminated. .../usr/bin/vax-netbsdelf-ld: error: lto-wrapper failed collect2: error: ld returned 1 exit status compiler exited with status 1 FAIL: gcc.dg/lto/pr69866 c_lto_pr69866_0.o-c_lto_pr69866_1.o link, -O0 -flto -fuse-linker-plugin -fno-fat-lto-objects (internal compiler error) Substitute `-flinker-output=pie' for `-flinker-output=exec' in the specs then unless `-no-pie' has also been used, preserving the original intent of emitting PIC code by default for executables while keeping the linker arrangement unchanged. The LTO compiler uses the `cc1' spec, so keep `cc1plus' unmodified. This makes code like: .L6: movab n,%r0 addl2 -8(%fp),%r0 pushl %r0 calls $1,foo addl2 %r0,-12(%fp) incl -8(%fp) .L5: be produced instead corresponding to the fragment quoted above, which is valid PIC code as it uses the PC-relative addressing mode denoted by the absence of a prefix to `n' (which can be redirected to GOT as required, by changing the addressing mode to PC-relative indirect in the operand specifier). Ideally we would instead default to the PIE model for executables, but that triggers a BFD bug where for a change the LTO wrapper is not used: .../usr/bin/vax-netbsdelf-ld: /tmp/ccV2sWQt.ltrans0.ltrans.o: warning: GOT addend of 3 to `n' does not match previous GOT addend of 0 FAIL: gcc.dg/lto/pr55660 c_lto_pr55660_0.o-c_lto_pr55660_1.o link, -O2 -flto -flto-partition=1to1 -fno-use-linker-plugin which is due to assembly code like: main: .word 0 subl2 $4,%sp movab n,%r0 movab n+3,%r2 clrl %r3 movb $98,%r1 .L4: and consequently object code like: 00000000 <main>: 0: 00 00 .word 0x0000 # Entry mask: < > 2: c2 04 5e subl2 $0x4,sp 5: 9e ef 00 00 movab b <main+0xb>,r0 9: 00 00 50 7: R_VAX_GOT32 n c: 9e ef 00 00 movab 12 <main+0x12>,r2 10: 00 00 52 e: R_VAX_GOT32 n+0x3 13: d4 53 clrf r3 15: 90 8f 62 51 movb $0x62,r1 being produced. This would be problematic for external `n', because we do not support multiple GOT entries for the same symbol referred to with different offsets in a single link unit. In this case however the LTO compiler correctly observes that `n' is defined by the executable and not preemptible and therefore no GOT entry will be made for it. Indeed a valid executable is produced: 00010548 <main>: 10548: 00 00 .word 0x0000 # Entry mask: < > 1054a: c2 04 5e subl2 $0x4,sp 1054d: 9e ef dd 14 movab 11a30 <n>,r0 10551: 00 00 50 10554: 9e ef d9 14 movab 11a33 <__bss_start>,r2 10558: 00 00 52 1055b: d4 53 clrf r3 1055d: 90 8f 62 51 movb $0x62,r1 despite the warning, but it would be rather bad to have users annoyed with this message from BFD, however harmless, especially as it triggers outside LTO compilations as well. Therefore this change is the best we can do until binutils have been fixed. References: [1] Jan Hubicka, "Getting LTO incremental linking work", <https://gcc.gnu.org/ml/gcc-patches/2015-11/msg02986.html> gcc/ * config/vax/elf.h (VAX_CC1_SPEC, VAX_CC1PLUS_SPEC): New macros. * config/vax/netbsd-elf.h (CC1_SPEC): Use VAX_CC1_SPEC rather than VAX_CC1_AND_CC1PLUS_SPEC. (CC1PLUS_SPEC): Use VAX_CC1PLUS_SPEC rather than VAX_CC1_AND_CC1PLUS_SPEC. --- Hi, This is currently getting through verification, which I expect to end by Wed. I would like to insert it into the original VAX series between 29/31 and 30/31 so as to get rid of the sole regression mentioned with the cover letter there, and obviously to get LTO compilations fixed. I'll be posting a BFD fix separately right away and then we can switch to PIE by default in the next development cycle, when we have had at least one binutils release made with the fix applied. Meanwhile, OK to apply this interim fix, once verification has completed? Maciej --- gcc/config/vax/elf.h | 10 ++++++++++ gcc/config/vax/netbsd-elf.h | 4 ++-- 2 files changed, 12 insertions(+), 2 deletions(-)