[all-commits] [llvm/llvm-project] 3bb1ce: Add a nop instruction if a section starts with lan...

[modiking via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3bb1ce2319034831f48278261ea1cdf5e6bbdd50
      https://github.com/llvm/llvm-project/commit/3bb1ce2319034831f48278261ea1cdf5e6bbdd50
  Author: ARCHIT SAXENA <archsaxe at fb.com>
  Date:   2022-07-22 (Fri, 22 Jul 2022)

  Changed paths:
    M llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h
    M llvm/lib/CodeGen/BasicBlockSections.cpp
    M llvm/lib/CodeGen/MachineFunctionSplitter.cpp
    M llvm/test/CodeGen/X86/machine-function-splitter.ll

  Log Message:
  -----------
  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", at 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 at 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", at 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 at PLT
```

Reviewed By: modimo, snehasish, rahmanl

Differential Revision: https://reviews.llvm.org/D130133