Description
Environment
- Arch: aarch64
- Clang Version: Clang 19 dev (e.g., clang version 19.0.0git)
- GCC Version: (e.g., gcc version 10.3)
- Compiler Flags: -fno-plt
Description:
I encountered an issue with the -fno-plt option in Clang 19 dev (development version) on aarch64. When compiling code with -fno-plt, Clang still generates PLT (Procedure Linkage Table) entries for function calls, whereas GCC behaves correctly and does not generate PLT entries.
This issue affects scenarios where direct GOT-based access is expected, and the presence of PLT entries may lead to unnecessary indirection and performance overhead.
Compile the following code with Clang 19 dev and -fno-plt
// gcc 1.c -shared -fno-plt -O0 -o libnoplt-clang.so -fPIC
// clang 1.c -shared -fno-plt -O0 -o libnoplt-gcc.so -fPIC
int bar(int a) {
return 10;
}
int foo(int a) {
return bar(a);
}
The Clang-compiled version generates PLT functions, while the GCC version does not.
// libnoplt-clang.so
Disassembly of section .plt:
0000000000000440 <.plt>:
440: a9bf7bf0 stp x16, x30, [sp, #-16]!
444: f00000f0 adrp x16, 1f000 <__FRAME_END__+0x1e934>
448: f947fe11 ldr x17, [x16, #4088]
44c: 913fe210 add x16, x16, #0xff8
450: d61f0220 br x17
454: d503201f nop
458: d503201f nop
45c: d503201f nop
0000000000000470 <bar@plt>:
470: 90000110 adrp x16, 20000 <__cxa_finalize@GLIBC_2.17>
474: f9400611 ldr x17, [x16, #8]
478: 91002210 add x16, x16, #0x8
47c: d61f0220 br x17
0000000000000588 <foo>:
588: d10083ff sub sp, sp, #0x20
58c: a9017bfd stp x29, x30, [sp, #16]
590: 910043fd add x29, sp, #0x10
594: b81fc3a0 stur w0, [x29, #-4]
598: b85fc3a0 ldur w0, [x29, #-4]
59c: 97ffffb5 bl 470 <bar@plt>
5a0: a9417bfd ldp x29, x30, [sp, #16]
5a4: 910083ff add sp, sp, #0x20
5a8: d65f03c0 ret
// libnoplt-gcc.so
00000000000005c4 <bar>:
5c4: d10043ff sub sp, sp, #0x10
5c8: b9000fe0 str w0, [sp, #12]
5cc: 52800140 mov w0, #0xa // #10
5d0: 910043ff add sp, sp, #0x10
5d4: d65f03c0 ret
00000000000005d8 <foo>:
5d8: a9be7bfd stp x29, x30, [sp, #-32]!
5dc: 910003fd mov x29, sp
5e0: b9001fe0 str w0, [sp, #28]
5e4: b9401fe0 ldr w0, [sp, #28]
5e8: f00000e1 adrp x1, 1f000 <__FRAME_END__+0x1e900>
5ec: f947e821 ldr x1, [x1, #4048]
5f0: d63f0020 blr x1
5f4: a8c27bfd ldp x29, x30, [sp], #32
5f8: d65f03c0 ret
Currently, LLVM has only been processed for the x86 and ARM + Mach-O architecture, but there are still problems with the aarch64 architecture.
relevant issue: Support fno-plt like gcc
relevant pr: Implement -fno-plt for SelectionDAG/GlobalISel