forked from OSchip/llvm-project
3bb1ce2319
This change adds a nop instruction if section starts with landing pad. This change is like [D73739](https://reviews.llvm.org/D73739) which avoids zero offset landing pad in basic block sections. Detailed description: The current machine functions splitter can create ˜sections which start with a landing pad themselves. This places landing pad at offset zero from LPStart. ``` .section .text.split.foo10,"ax",@progbits foo10.cold: # %lpad .cfi_startproc .cfi_personality 3, __gxx_personality_v0 .cfi_lsda 3, .Lexception5 .cfi_def_cfa %rsp, 16 .Ltmp11: <--- This is a Landing pad and also LP Start as it is start of this section movq %rax, %rdi <--- first instruction is at offest 0 from LPStart callq _Unwind_Resume@PLT ``` This will cause landing pad entries to become zero (.Ltmp11-foo10.cold) ``` .Lcst_begin4: .uleb128 .Ltmp9-.Lfunc_begin2 # >> Call Site 1 << .uleb128 .Ltmp10-.Ltmp9 # Call between .Ltmp9 and .Ltmp10 .uleb128 .Ltmp11-foo10.cold <---This is zero # jumps to .Ltmp11 .byte 3 # On action: 2 .uleb128 .Ltmp10-.Lfunc_begin2 # >> Call Site 2 << .uleb128 .Lfunc_end9-.Ltmp10 # Call between .Ltmp10 and .Lfunc_end9 .byte 0 # has no landing pad .byte 0 # On action: cleanup .p2align 2 ``` The C++ ABI somehow assumes that no landing pads point directly to LPStart (which works in the normal case since the function begin is never a landing pad), and uses LP.offset = 0 to specify no landing pad. This change adds a nop instruction at start of such sections so that such a case could be avoided. Output: ``` .section .text.split.foo10,"ax",@progbits foo10.cold: # %lpad .cfi_startproc .cfi_personality 3, __gxx_personality_v0 .cfi_lsda 3, .Lexception5 .cfi_def_cfa %rsp, 16 nop <--- new instruction that is added .Ltmp11: movq %rax, %rdi callq _Unwind_Resume@PLT ``` Reviewed By: modimo, snehasish, rahmanl Differential Revision: https://reviews.llvm.org/D130133 |
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README.txt
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