https://sourceware.org/bugzilla/show_bug.cgi?id=19252
Bug ID: 19252
Summary: ld rewrites instructions even when asked not to, in
x86-64
Product: binutils
Version: 2.25
Status: NEW
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: srk31 at srcf dot ucam.org
Target Milestone: ---
Created attachment 8785
--> https://sourceware.org/bugzilla/attachment.cgi?id=8785&action=edit
Test case
To reduce use of the GOT, ld sometimes simplifies GOT- into non-GOT relocs on
x86-64. It does so by (perhaps among others) turning R_X86_64_GOTPCREL to
R_X86_64_PC32, and changing the relocated instruction from mov to lea.
I think it should either
- only do this when --relax is passed, or
- only do this when -O is passed, or
- not do this if --no-relax is passed, or
- not do this if -O0 is passed.
It should also have something in the manual to say when it does this.
(Rationale: building a tool that checks correctness properties of a link output
is much more difficult if ld futzes around with instructions. A naive user
would not expect ld to change instructions at all, except as directed by
relocs. Ideally ld would document every additional instruction-level
transformation it makes, in the "Machine Dependent" chapter, and would provide
a way to turn them off.)
The attached tarball illustrates. The first few instructions of __uClibc_main
in the input libc.a are as follows.
00000000000000d2 <__uClibc_main>:
d2: 41 55 push %r13
d4: 41 54 push %r12
d6: 4d 89 c4 mov %r8,%r12
d9: 55 push %rbp
da: 53 push %rbx
db: 48 89 cd mov %rcx,%rbp
de: 48 81 ec 88 01 00 00 sub $0x188,%rsp
e5: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax
e8: R_X86_64_GOTPCREL __libc_stack_end-0x4
... whereas in the output (linked with ld -q --no-relax -O0) we see the
following.
0000000000400349 <__uClibc_main>:
400349: 41 55 push %r13
40034b: 41 54 push %r12
40034d: 4d 89 c4 mov %r8,%r12
400350: 55 push %rbp
400351: 53 push %rbx
400352: 48 89 cd mov %rcx,%rbp
400355: 48 81 ec 88 01 00 00 sub $0x188,%rsp
40035c: 48 8d 05 3d 1e 20 00 lea 0x201e3d(%rip),%rax
40035f: R_X86_64_PC32 __libc_stack_end-0x4
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