diff options
| author | ARCHIT SAXENA <archsaxe@fb.com> | 2022-07-22 15:02:16 -0700 |
|---|---|---|
| committer | modimo <modimo@fb.com> | 2022-07-22 15:20:10 -0700 |
| commit | 3bb1ce2319034831f48278261ea1cdf5e6bbdd50 (patch) | |
| tree | 466456e3f13e9309c9ffd392aafb9b7c7350fe91 /llvm/include | |
| parent | 2ebfda2417e9cead344b020c1b1b7d3344893291 (diff) | |
| download | llvm-3bb1ce2319034831f48278261ea1cdf5e6bbdd50.zip llvm-3bb1ce2319034831f48278261ea1cdf5e6bbdd50.tar.gz llvm-3bb1ce2319034831f48278261ea1cdf5e6bbdd50.tar.bz2 | |
Add a nop instruction if a section starts with landing pad for function splitter
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
Diffstat (limited to 'llvm/include')
| -rw-r--r-- | llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h b/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h index d8da3be..d43f399 100644 --- a/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h +++ b/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h @@ -25,6 +25,8 @@ using MachineBasicBlockComparator = void sortBasicBlocksAndUpdateBranches(MachineFunction &MF, MachineBasicBlockComparator MBBCmp); +void avoidZeroOffsetLandingPad(MachineFunction &MF); + } // end namespace llvm #endif // LLVM_CODEGEN_BASICBLOCKSECTIONUTILS_H |