[llvm] [BranchFolding] Kill common hoisted debug instructions (PR #140091)
Orlando Cazalet-Hyams via llvm-commits
llvm-commits at lists.llvm.org
Fri Jul 18 08:57:52 PDT 2025
https://github.com/OCHyams updated https://github.com/llvm/llvm-project/pull/140091
>From e831e12ab09a3793582f111fe4ac33933b626e46 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Thu, 15 May 2025 14:08:45 +0100
Subject: [PATCH 1/7] [BranchFolding] Kill common hoisted debug instructions
branch-folder hoists common instructions from TBB and FBB into their
pred. Without this patch it achieves this by splicing the instructions from TBB
and deleting the common ones in FBB. That moves the debug locations and debug
instructions from TBB into the pred without modification, which is not
ideal. Debug locations are handled in #140063.
This patch handles debug instructions - in the simplest way possible, which is
to just kill (undef) them. We kill and hoist the ones in FBB as well as TBB
because otherwise the fact there's an assignment on the code path is deleted
(which might lead to a prior location extending further than it should).
There's possibly something we could do to preserve some variable locations in
some cases, but this is the easiest not-incorrect thing to do.
Note I had to replace the constant DBG_VALUEs to use registers in the test- it
turns out setDebugValueUndef doesn't undef constant DBG_VALUEs... which feels
wrong to me, but isn't something I want to touch right now.
---
llvm/lib/CodeGen/BranchFolding.cpp | 44 +++++++++++++++++--
llvm/test/DebugInfo/X86/branch-folder-dbg.mir | 34 ++++++++------
2 files changed, 60 insertions(+), 18 deletions(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 3b3e7a418feb5..85712833d844a 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -25,6 +25,7 @@
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/BranchFoldingPass.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
#include "llvm/CodeGen/MBFIWrapper.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
@@ -2083,19 +2084,53 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
if (TBB == FBB) {
MBB->splice(Loc, TBB, TBB->begin(), TIB);
} else {
+ // Merge the debug locations, and hoist and kill the debug instructions from
+ // both branches. FIXME: We could probably try harder to preserve some debug
+ // instructions (but at least this isn't producing wrong locations).
+ MachineIRBuilder MIRBuilder(*MBB, Loc);
+ auto HoistAndKillDbgInstr =
+ [&MIRBuilder](MachineBasicBlock::iterator DI,
+ MachineBasicBlock::iterator InsertBefore) {
+ assert(DI->isDebugInstr() && "Expected a debug instruction");
+ if (DI->isDebugRef()) {
+ MIRBuilder.setDebugLoc(DI->getDebugLoc());
+ MIRBuilder.buildDirectDbgValue(0, DI->getDebugVariable(),
+ DI->getDebugExpression());
+ return;
+ }
+
+ DI->setDebugValueUndef();
+ DI->moveBefore(&*InsertBefore);
+ };
+
// TIB and FIB point to the end of the regions to hoist/merge in TBB and
// FBB.
MachineBasicBlock::iterator FE = FIB;
MachineBasicBlock::iterator FI = FBB->begin();
for (MachineBasicBlock::iterator TI :
make_early_inc_range(make_range(TBB->begin(), TIB))) {
- // Move debug instructions and pseudo probes without modifying them.
- // FIXME: This is the wrong thing to do for debug locations, which
- // should at least be killed (and hoisted from BOTH blocks).
- if (TI->isDebugOrPseudoInstr()) {
+
+ // Hoist and kill debug instructions from FBB. Skip over pseudo
+ // instructions. After this loop FI points to the next non-debug
+ // instruction to hoist (checked in assert after the TBB debug
+ // instruction handling code).
+ while (FI->isDebugOrPseudoInstr()) {
+ assert(FI != FE && "Unexpected end of FBB range");
+ MachineBasicBlock::iterator FINext = std::next(FI);
+ HoistAndKillDbgInstr(FI, Loc);
+ FI = FINext;
+ }
+
+ // Move pseudo probes without modifying them.
+ if (TI->isPseudoProbe()) {
TI->moveBefore(&*Loc);
continue;
}
+ // Kill debug instructions before moving.
+ if (TI->isDebugInstr()) {
+ HoistAndKillDbgInstr(TI, Loc);
+ continue;
+ }
// Get the next non-meta instruction in FBB.
FI = skipDebugInstructionsForward(FI, FE, false);
@@ -2111,6 +2146,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
++FI;
}
}
+ assert(FIB->getParent() == FBB && "Unexpectedly moved FIB?");
FBB->erase(FBB->begin(), FIB);
if (UpdateLiveIns)
diff --git a/llvm/test/DebugInfo/X86/branch-folder-dbg.mir b/llvm/test/DebugInfo/X86/branch-folder-dbg.mir
index 5c38fd2a43829..831b263fdccd4 100644
--- a/llvm/test/DebugInfo/X86/branch-folder-dbg.mir
+++ b/llvm/test/DebugInfo/X86/branch-folder-dbg.mir
@@ -1,16 +1,21 @@
# RUN: llc %s --start-before=branch-folder --stop-after=branch-folder -o - | FileCheck %s
## Check that common instructions hoisted from `if.then` and `if.else` into
-## common pred `entry` get merged debug locations.
-
-## FIXME: The debug instructions handling here is wrong.
+## common pred `entry` get merged debug locations. The debug instructions from
+## both branches should get hoisted and killed.
+##
+## The MIR debug instructions have been modified by hand in order to check they
+## can be killed.
# CHECK: bb.0
# CHECK: CALL64pcrel32 @f, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
-## --- Start splice from bb.2.if.else ---
-# CHECK-NEXT: DBG_VALUE 2, $noreg, ![[#]], !DIExpression(), debug-location ![[#]]
-# CHECK-NEXT: $edi = MOV32r0 implicit-def dead $eflags, debug-location !DILocation(line: 0, scope: ![[#]])
-## --- End splice --------------
+## --- Start splice from bb.2.if.else (and debug instructions from bb.1.if.then) ---
+# CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(), debug-location ![[#]]
+# CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(), debug-location ![[#]]
+# CHECK-NEXT: $edi = MOV32r0 implicit-def dead $eflags, debug-instr-number 2, debug-location !DILocation(line: 0, scope: ![[#]])
+# CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(DW_OP_LLVM_arg, 0), debug-location ![[#]]
+# CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(DW_OP_LLVM_arg, 0), debug-location ![[#]]
+## --- End splice ------------------------------------------------------------------
# CHECK-NEXT: TEST64rr killed renamable $rax, renamable $rax, implicit-def $eflags
# CHECK-NEXT: JCC_1 %bb.2, 9, implicit killed $eflags
# CHECK: bb.1
@@ -73,6 +78,8 @@
...
---
name: g
+tracksRegLiveness: true
+isSSA: false
body: |
bb.0 (%ir-block.0):
successors: %bb.1(0x40000000), %bb.2(0x40000000)
@@ -86,22 +93,21 @@ body: |
bb.1.if.then:
successors: %bb.3(0x80000000)
-
- DBG_VALUE 0, $noreg, !11, !DIExpression(), debug-location !13
- $edi = MOV32r0 implicit-def dead $eflags, debug-location !14
+ DBG_VALUE $esi, $noreg, !11, !DIExpression(), debug-location !13
+ $edi = MOV32r0 implicit-def dead $eflags, debug-instr-number 1, debug-location !14
+ DBG_INSTR_REF !11, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0), debug-location !13
$esi = MOV32r0 implicit-def dead $eflags, debug-location !14
CALL64pcrel32 target-flags(x86-plt) @_Z3fooii, csr_64, implicit $rsp, implicit $ssp, implicit killed $edi, implicit killed $esi, implicit-def $rsp, implicit-def $ssp, debug-location !14
- DBG_VALUE 1, $noreg, !11, !DIExpression(), debug-location !13
JMP_1 %bb.3, debug-location !15
bb.2.if.else:
successors: %bb.3(0x80000000)
- DBG_VALUE 2, $noreg, !11, !DIExpression(), debug-location !13
- $edi = MOV32r0 implicit-def dead $eflags, debug-location !16
+ DBG_VALUE $esp, $noreg, !11, !DIExpression(), debug-location !13
+ $edi = MOV32r0 implicit-def dead $eflags, debug-instr-number 2, debug-location !16
+ DBG_INSTR_REF !11, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(2, 0), debug-location !13
$esi = MOV32ri 1, debug-location !16
CALL64pcrel32 target-flags(x86-plt) @_Z3barii, csr_64, implicit $rsp, implicit $ssp, implicit killed $edi, implicit killed $esi, implicit-def $rsp, implicit-def $ssp, debug-location !16
- DBG_VALUE 3, $noreg, !11, !DIExpression(), debug-location !13
bb.3.if.end:
$eax = MOV32ri 2
>From c6562bf345f426714646c73bc294bd3172d0cfff Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Thu, 29 May 2025 18:12:03 +0100
Subject: [PATCH 2/7] replace MachineIRBuilder with BuildMI
---
llvm/lib/CodeGen/BranchFolding.cpp | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 85712833d844a..0b31db878803d 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -25,7 +25,6 @@
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/BranchFoldingPass.h"
-#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
#include "llvm/CodeGen/MBFIWrapper.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
@@ -49,6 +48,8 @@
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/LaneBitmask.h"
+#include "llvm/MC/MCInstrDesc.h"
+#include "llvm/MC/MCInstrInfo.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
@@ -2087,15 +2088,18 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
// Merge the debug locations, and hoist and kill the debug instructions from
// both branches. FIXME: We could probably try harder to preserve some debug
// instructions (but at least this isn't producing wrong locations).
- MachineIRBuilder MIRBuilder(*MBB, Loc);
+ MachineInstrBuilder MIRBuilder(*MBB->getParent(), Loc);
auto HoistAndKillDbgInstr =
- [&MIRBuilder](MachineBasicBlock::iterator DI,
- MachineBasicBlock::iterator InsertBefore) {
+ [MBB](MachineBasicBlock::iterator DI,
+ MachineBasicBlock::iterator InsertBefore) {
assert(DI->isDebugInstr() && "Expected a debug instruction");
if (DI->isDebugRef()) {
- MIRBuilder.setDebugLoc(DI->getDebugLoc());
- MIRBuilder.buildDirectDbgValue(0, DI->getDebugVariable(),
- DI->getDebugExpression());
+ const TargetInstrInfo *TII =
+ MBB->getParent()->getSubtarget().getInstrInfo();
+ const MCInstrDesc &DBGV = TII->get(TargetOpcode::DBG_VALUE);
+ DI = BuildMI(*MBB->getParent(), DI->getDebugLoc(), DBGV, false, 0,
+ DI->getDebugVariable(), DI->getDebugExpression());
+ MBB->insert(InsertBefore, &*DI);
return;
}
>From a2fa3c2cd18cba35216e0b010cc37b58b697e761 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Thu, 29 May 2025 18:13:50 +0100
Subject: [PATCH 3/7] use capture instead of param
---
llvm/lib/CodeGen/BranchFolding.cpp | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 0b31db878803d..2f16e960645cf 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -2090,8 +2090,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
// instructions (but at least this isn't producing wrong locations).
MachineInstrBuilder MIRBuilder(*MBB->getParent(), Loc);
auto HoistAndKillDbgInstr =
- [MBB](MachineBasicBlock::iterator DI,
- MachineBasicBlock::iterator InsertBefore) {
+ [MBB, Loc](MachineBasicBlock::iterator DI) {
assert(DI->isDebugInstr() && "Expected a debug instruction");
if (DI->isDebugRef()) {
const TargetInstrInfo *TII =
@@ -2099,12 +2098,12 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
const MCInstrDesc &DBGV = TII->get(TargetOpcode::DBG_VALUE);
DI = BuildMI(*MBB->getParent(), DI->getDebugLoc(), DBGV, false, 0,
DI->getDebugVariable(), DI->getDebugExpression());
- MBB->insert(InsertBefore, &*DI);
+ MBB->insert(Loc, &*DI);
return;
}
DI->setDebugValueUndef();
- DI->moveBefore(&*InsertBefore);
+ DI->moveBefore(&*Loc);
};
// TIB and FIB point to the end of the regions to hoist/merge in TBB and
@@ -2121,7 +2120,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
while (FI->isDebugOrPseudoInstr()) {
assert(FI != FE && "Unexpected end of FBB range");
MachineBasicBlock::iterator FINext = std::next(FI);
- HoistAndKillDbgInstr(FI, Loc);
+ HoistAndKillDbgInstr(FI);
FI = FINext;
}
@@ -2132,7 +2131,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
}
// Kill debug instructions before moving.
if (TI->isDebugInstr()) {
- HoistAndKillDbgInstr(TI, Loc);
+ HoistAndKillDbgInstr(TI);
continue;
}
>From 7ed3eb4004f09d746b9f270726efc1809dc1b341 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Thu, 29 May 2025 18:15:58 +0100
Subject: [PATCH 4/7] format
---
llvm/lib/CodeGen/BranchFolding.cpp | 29 ++++++++++++++---------------
1 file changed, 14 insertions(+), 15 deletions(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 2f16e960645cf..3a678425e6f26 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -2089,22 +2089,21 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
// both branches. FIXME: We could probably try harder to preserve some debug
// instructions (but at least this isn't producing wrong locations).
MachineInstrBuilder MIRBuilder(*MBB->getParent(), Loc);
- auto HoistAndKillDbgInstr =
- [MBB, Loc](MachineBasicBlock::iterator DI) {
- assert(DI->isDebugInstr() && "Expected a debug instruction");
- if (DI->isDebugRef()) {
- const TargetInstrInfo *TII =
- MBB->getParent()->getSubtarget().getInstrInfo();
- const MCInstrDesc &DBGV = TII->get(TargetOpcode::DBG_VALUE);
- DI = BuildMI(*MBB->getParent(), DI->getDebugLoc(), DBGV, false, 0,
- DI->getDebugVariable(), DI->getDebugExpression());
- MBB->insert(Loc, &*DI);
- return;
- }
+ auto HoistAndKillDbgInstr = [MBB, Loc](MachineBasicBlock::iterator DI) {
+ assert(DI->isDebugInstr() && "Expected a debug instruction");
+ if (DI->isDebugRef()) {
+ const TargetInstrInfo *TII =
+ MBB->getParent()->getSubtarget().getInstrInfo();
+ const MCInstrDesc &DBGV = TII->get(TargetOpcode::DBG_VALUE);
+ DI = BuildMI(*MBB->getParent(), DI->getDebugLoc(), DBGV, false, 0,
+ DI->getDebugVariable(), DI->getDebugExpression());
+ MBB->insert(Loc, &*DI);
+ return;
+ }
- DI->setDebugValueUndef();
- DI->moveBefore(&*Loc);
- };
+ DI->setDebugValueUndef();
+ DI->moveBefore(&*Loc);
+ };
// TIB and FIB point to the end of the regions to hoist/merge in TBB and
// FBB.
>From 82c3f6030d8e171d374c9a880128b062e9b7fcb9 Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Fri, 18 Jul 2025 14:47:45 +0100
Subject: [PATCH 5/7] rm unecessary includes
---
llvm/lib/CodeGen/BranchFolding.cpp | 2 --
1 file changed, 2 deletions(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 3a678425e6f26..a2ac9643ce2af 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -48,8 +48,6 @@
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/LaneBitmask.h"
-#include "llvm/MC/MCInstrDesc.h"
-#include "llvm/MC/MCInstrInfo.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
>From cd41d6670157cd917becf45ad01e8235983e953d Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Fri, 18 Jul 2025 16:51:21 +0100
Subject: [PATCH 6/7] Remove unecessary pseudo probe handling
---
llvm/lib/CodeGen/BranchFolding.cpp | 21 +++++++++------------
1 file changed, 9 insertions(+), 12 deletions(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index a2ac9643ce2af..1234a9bac713e 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -2109,31 +2109,28 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
MachineBasicBlock::iterator FI = FBB->begin();
for (MachineBasicBlock::iterator TI :
make_early_inc_range(make_range(TBB->begin(), TIB))) {
-
- // Hoist and kill debug instructions from FBB. Skip over pseudo
- // instructions. After this loop FI points to the next non-debug
- // instruction to hoist (checked in assert after the TBB debug
- // instruction handling code).
- while (FI->isDebugOrPseudoInstr()) {
+ // Hoist and kill debug instructions from FBB. After this loop FI points
+ // to the next non-debug instruction to hoist (checked in assert after the
+ // TBB debug instruction handling code).
+ while (FI->isDebugInstr()) {
assert(FI != FE && "Unexpected end of FBB range");
MachineBasicBlock::iterator FINext = std::next(FI);
HoistAndKillDbgInstr(FI);
FI = FINext;
}
- // Move pseudo probes without modifying them.
- if (TI->isPseudoProbe()) {
- TI->moveBefore(&*Loc);
- continue;
- }
// Kill debug instructions before moving.
if (TI->isDebugInstr()) {
HoistAndKillDbgInstr(TI);
continue;
}
- // Get the next non-meta instruction in FBB.
+ // If FI is a debug instruction, skip forward to the next non-debug
+ // instruction.
FI = skipDebugInstructionsForward(FI, FE, false);
+ // Pseudo probes are excluded from the range when identifying foldable
+ // instructions, so we don't expect to see one now.
+ assert(!TI->isPseudoProbe() && "Unexpected psuedo probe in range");
// NOTE: The loop above checks CheckKillDead but we can't do that here as
// it modifies some kill markers after the check.
assert(TI->isIdenticalTo(*FI, MachineInstr::CheckDefs) &&
>From cbedb86a0a666fd63c50b90386d38e50f38f306e Mon Sep 17 00:00:00 2001
From: Orlando Cazalet-Hyams <orlando.hyams at sony.com>
Date: Fri, 18 Jul 2025 16:57:38 +0100
Subject: [PATCH 7/7] spelllling
---
llvm/lib/CodeGen/BranchFolding.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 1234a9bac713e..99d5443416fe4 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -2130,7 +2130,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
FI = skipDebugInstructionsForward(FI, FE, false);
// Pseudo probes are excluded from the range when identifying foldable
// instructions, so we don't expect to see one now.
- assert(!TI->isPseudoProbe() && "Unexpected psuedo probe in range");
+ assert(!TI->isPseudoProbe() && "Unexpected pseudo probe in range");
// NOTE: The loop above checks CheckKillDead but we can't do that here as
// it modifies some kill markers after the check.
assert(TI->isIdenticalTo(*FI, MachineInstr::CheckDefs) &&
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