[RFC PATCH 2/2] x86: Alternative <sys/platform/x86.h> implementation

[H.J. Lu]

On Wed, Dec 23, 2020 at 11:15:01PM +0100, GNU C Library wrote:
> This implementation uses a data symbol containing a pointer
> to the bitmask array and an array length, not a function call.
> The expectation is that this is the final link-time ABI for
> this feature.  (The run-time ABI will have to change once more,
> to support use of this facility in IFUNC resolvers.)
> 
> The __libc_vars initialization mechanism is used to set up
> the copy in libc.so.6 before relocation, so that it can be used
> by IFUNC resolvers.
> 
> Usage of the C preprocessor is greatly reduced, making it easier
> to wrap this functionality in other languages.
> 
> This is still a preview.  There are further cleanups possible,
> including removal of the function symbol.  The manual still needs
> updating, and there are a few overlong lines.  I'd like to receive
> feedback if this is the direction in which we want to move.
> 
> I think it should be possible to hack in IFUNC resolver support using a
> custom dynamic section entry that points to a hidden __x86_cpu_array
> variable.  It would be cleaner to use a new run-time-only relocation for
> the initialization.  The dynamic section hack would not work with
> --gc-sections, for instance.
> 

It doesn't work:

[hjl@gnu-cfl-2 build-x86_64-linux]$ readelf -rW math/libm.so

Relocation section '.rela.dyn' at offset 0xe120 contains 13 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000140d88  0000000000000008 R_X86_64_RELATIVE                         f2b0
0000000000140d90  0000000000000008 R_X86_64_RELATIVE                         f270
0000000000140d98  0000000000000008 R_X86_64_RELATIVE                         140d98
0000000000140fb0  0000000200000006 R_X86_64_GLOB_DAT      0000000000000000 _ITM_deregisterTMCloneTable + 0
0000000000140fb8  0000000300000012 R_X86_64_TPOFF64       0000000000000000 errno@GLIBC_PRIVATE + 0
0000000000140fc0  000003b200000006 R_X86_64_GLOB_DAT      00000000001410f0 _LIB_VERSION@GLIBC_2.2.5 + 0
0000000000140fc8  0000000a00000006 R_X86_64_GLOB_DAT      0000000000000000 __x86_cpu_array@GLIBC_2.33 + 0
0000000000140fd0  0000000b00000006 R_X86_64_GLOB_DAT      0000000000000000 __gmon_start__ + 0
0000000000140fd8  0000020800000006 R_X86_64_GLOB_DAT      00000000001410f8 signgam@@GLIBC_2.2.5 + 0
0000000000140fe0  0000021200000006 R_X86_64_GLOB_DAT      00000000001410f8 __signgam@@GLIBC_2.23 + 0
0000000000140fe8  0000000f00000006 R_X86_64_GLOB_DAT      0000000000000000 _ITM_registerTMCloneTable + 0
0000000000140ff0  0000001000000006 R_X86_64_GLOB_DAT      0000000000000000 __cxa_finalize@GLIBC_2.2.5 + 0
0000000000140ff8  0000001100000006 R_X86_64_GLOB_DAT      0000000000000000 stderr@GLIBC_2.2.5 + 0

Relocation section '.rela.plt' at offset 0xe258 contains 27 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000141018  0000000100000007 R_X86_64_JUMP_SLOT     0000000000000000 __strtold_nan@GLIBC_PRIVATE + 0
0000000000141030  0000000400000007 R_X86_64_JUMP_SLOT     0000000000000000 qsort@GLIBC_2.2.5 + 0
0000000000141038  0000000500000007 R_X86_64_JUMP_SLOT     0000000000000000 __strtod_nan@GLIBC_PRIVATE + 0
0000000000141040  0000000600000007 R_X86_64_JUMP_SLOT     0000000000000000 __strtof128_nan@GLIBC_PRIVATE + 0
0000000000141048  0000000700000007 R_X86_64_JUMP_SLOT     0000000000000000 __assert_fail@GLIBC_2.2.5 + 0
0000000000141050  0000000800000007 R_X86_64_JUMP_SLOT     0000000000000000 fputs@GLIBC_2.2.5 + 0
0000000000141060  0000000900000007 R_X86_64_JUMP_SLOT     0000000000000000 memset@GLIBC_2.2.5 + 0
0000000000141068  0000018f00000007 R_X86_64_JUMP_SLOT     000000000000f2c0 matherr@GLIBC_2.2.5 + 0
00000000001410a8  0000000c00000007 R_X86_64_JUMP_SLOT     0000000000000000 __strtof_nan@GLIBC_PRIVATE + 0
00000000001410b8  0000000d00000007 R_X86_64_JUMP_SLOT     0000000000000000 memmove@GLIBC_2.2.5 + 0
00000000001410d0  0000000e00000007 R_X86_64_JUMP_SLOT     0000000000000000 fwrite@GLIBC_2.2.5 + 0
00000000001410e8  0000000000000025 R_X86_64_IRELATIVE                        46710
00000000001410e0  0000000000000025 R_X86_64_IRELATIVE                        29df0
00000000001410d8  0000000000000025 R_X86_64_IRELATIVE                        42430
00000000001410c8  0000000000000025 R_X86_64_IRELATIVE                        33a60
00000000001410c0  0000000000000025 R_X86_64_IRELATIVE                        44f20
00000000001410b0  0000000000000025 R_X86_64_IRELATIVE                        48390
00000000001410a0  0000000000000025 R_X86_64_IRELATIVE                        45420
0000000000141098  0000000000000025 R_X86_64_IRELATIVE                        2da30
0000000000141090  0000000000000025 R_X86_64_IRELATIVE                        482b0
0000000000141088  0000000000000025 R_X86_64_IRELATIVE                        2d170
0000000000141080  0000000000000025 R_X86_64_IRELATIVE                        257e0
0000000000141078  0000000000000025 R_X86_64_IRELATIVE                        47c50
0000000000141070  0000000000000025 R_X86_64_IRELATIVE                        25780
0000000000141058  0000000000000025 R_X86_64_IRELATIVE                        4ea50
0000000000141028  0000000000000025 R_X86_64_IRELATIVE                        298b0
0000000000141020  0000000000000025 R_X86_64_IRELATIVE                        339f0

R_X86_64_IRELATIVE relocations depend on:

0000000000140fc8  0000000a00000006 R_X86_64_GLOB_DAT      0000000000000000 __x86_cpu_array@GLIBC_2.33 + 0

which is processed after R_X86_64_IRELATIVE.


H.J.