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[PATCH 4/5] x86-64: further tighten convert-load-reloc checking

Jan Beulich jbeulich@suse.com
Tue Feb 4 10:04:22 GMT 2025 
On 03.02.2025 23:41, H.J. Lu wrote:
> On Mon, Feb 3, 2025 at 7:41 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> REX2.M affects what insn we're actually dealing with, so we better check
>> this to avoid transforming (future) insns we must not touch.
>>
>> --- a/bfd/elf64-x86-64.c
>> +++ b/bfd/elf64-x86-64.c
>> @@ -2282,14 +2282,14 @@ elf_x86_64_convert_load_reloc (bfd *abfd
>>           if (to_reloc_pc32)
>>             return true;
>>
>> -         if (opcode == 0x85)
>> +         if (opcode == 0x85 && !(rex2 & (REX2_M << 4)))
>>             {
>>               /* Convert "test %reg, foo@GOTPCREL(%rip)" to
>>                  "test $foo, %reg".  */
>>               modrm = 0xc0 | (modrm & 0x38) >> 3;
>>               opcode = 0xf7;
>>             }
>> -         else if ((opcode | 0x38) == 0x3b)
>> +         else if ((opcode | 0x38) == 0x3b && !(rex2 & (REX2_M << 4)))
>>             {
>>               /* Convert "binop foo@GOTPCREL(%rip), %reg" to
>>                  "binop $foo, %reg".  */
>>
> 
> Please add a testcase to show it makes a difference.

Hmm, not sure how such a testcase would look like. At least some of
the involved opcodes have no meaning (yet) with REX2. But maybe I
can construct something. Still I view it as unreasonable that such
obvious omissions in earlier changes can't be corrected without
investing a lot of time in trying to make up a situation where
things would fail. Once again: Proof of _no failure_ should have
been added when these optimizations were introduced. And that proof
should have been extended when APX support was added. What you're
effectively doing is to ask me to cover for earlier omissions.

Jan
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