[llvm] 3bb1ce2 - Add a nop instruction if a section starts with landing pad for function splitter
via llvm-commits
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Fri Jul 22 15:20:25 PDT 2022
Author: ARCHIT SAXENA
Date: 2022-07-22T15:20:10-07:00
New Revision: 3bb1ce2319034831f48278261ea1cdf5e6bbdd50
URL: https://github.com/llvm/llvm-project/commit/3bb1ce2319034831f48278261ea1cdf5e6bbdd50
DIFF: https://github.com/llvm/llvm-project/commit/3bb1ce2319034831f48278261ea1cdf5e6bbdd50.diff
LOG: 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
Added:
Modified:
llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h
llvm/lib/CodeGen/BasicBlockSections.cpp
llvm/lib/CodeGen/MachineFunctionSplitter.cpp
llvm/test/CodeGen/X86/machine-function-splitter.ll
Removed:
################################################################################
diff --git a/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h b/llvm/include/llvm/CodeGen/BasicBlockSectionUtils.h
index d8da3be0cd4c3..d43f399b2c310 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
diff --git a/llvm/lib/CodeGen/BasicBlockSections.cpp b/llvm/lib/CodeGen/BasicBlockSections.cpp
index 0d26e34b5c3cb..958212a0e448d 100644
--- a/llvm/lib/CodeGen/BasicBlockSections.cpp
+++ b/llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -268,8 +268,8 @@ void llvm::sortBasicBlocksAndUpdateBranches(
// If the exception section begins with a landing pad, that landing pad will
// assume a zero offset (relative to @LPStart) in the LSDA. However, a value of
// zero implies "no landing pad." This function inserts a NOP just before the EH
-// pad label to ensure a nonzero offset. Returns true if padding is not needed.
-static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
+// pad label to ensure a nonzero offset.
+void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
for (auto &MBB : MF) {
if (MBB.isBeginSection() && MBB.isEHPad()) {
MachineBasicBlock::iterator MI = MBB.begin();
@@ -278,10 +278,8 @@ static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
MCInst Nop = MF.getSubtarget().getInstrInfo()->getNop();
BuildMI(MBB, MI, DebugLoc(),
MF.getSubtarget().getInstrInfo()->get(Nop.getOpcode()));
- return false;
}
}
- return true;
}
// This checks if the source of this function has drifted since this binary was
diff --git a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
index 867a7ed584b21..3e1aace855a57 100644
--- a/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
+++ b/llvm/lib/CodeGen/MachineFunctionSplitter.cpp
@@ -146,7 +146,7 @@ bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
return X.getSectionID().Type < Y.getSectionID().Type;
};
llvm::sortBasicBlocksAndUpdateBranches(MF, Comparator);
-
+ llvm::avoidZeroOffsetLandingPad(MF);
return true;
}
diff --git a/llvm/test/CodeGen/X86/machine-function-splitter.ll b/llvm/test/CodeGen/X86/machine-function-splitter.ll
index e4743e98c64cc..8e428515a4b5b 100644
--- a/llvm/test/CodeGen/X86/machine-function-splitter.ll
+++ b/llvm/test/CodeGen/X86/machine-function-splitter.ll
@@ -242,6 +242,28 @@ define void @foo9(i1 zeroext %0) nounwind #0 !prof !14 {
ret void
}
+define i32 @foo10(i1 zeroext %0) personality ptr @__gxx_personality_v0 !prof !14 {
+;; Check that nop is inserted just before the EH pad if it's beginning a section.
+; MFS-DEFAULTS-LABEL: foo10
+; MFS-DEFAULTS-LABEL: callq baz
+; MFS-DEFAULTS: .section .text.split.foo10,"ax", at progbits
+; MFS-DEFAULTS-NEXT: foo10.cold:
+; MFS-DEFAULTS: nop
+; MFS-DEFAULTS: callq _Unwind_Resume at PLT
+entry:
+ invoke void @_Z1fv()
+ to label %try.cont unwind label %lpad, !prof !17
+
+lpad:
+ %1 = landingpad { ptr, i32 }
+ cleanup
+ catch ptr @_ZTIi
+ resume { ptr, i32 } %1
+
+try.cont:
+ %2 = call i32 @baz()
+ ret i32 %2
+}
declare i32 @bar()
declare i32 @baz()
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