[llvm] [BranchFolding] Kill common hoisted debug instructions (PR #149999)
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Tue Jul 22 04:14:50 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-debuginfo
Author: Orlando Cazalet-Hyams (OCHyams)
<details>
<summary>Changes</summary>
Mirror of https://github.com/llvm/llvm-project/pull/140091 with an assertion failure fixed in the latest commit as a DBG_PHI was tripping a checking in `setDebugValueUndef` - @<!-- -->jmorse does the last commit look alright to you?
Reproducer shared here - https://github.com/llvm/llvm-project/pull/140091#issuecomment-3097416136
---
Full diff: https://github.com/llvm/llvm-project/pull/149999.diff
2 Files Affected:
- (modified) llvm/lib/CodeGen/BranchFolding.cpp (+44-6)
- (modified) llvm/test/DebugInfo/X86/branch-folder-dbg.mir (+24-13)
``````````diff
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 3b3e7a418feb5..1cb0a239c3d48 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -2083,22 +2083,59 @@ 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).
+ 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;
+ }
+ // Deleting a DBG_PHI results in an undef at the referenced DBG_INSTR_REF.
+ if (DI->isDebugPHI()) {
+ DI->eraseFromParent();
+ return;
+ }
+
+ DI->setDebugValueUndef();
+ DI->moveBefore(&*Loc);
+ };
+
// 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()) {
- TI->moveBefore(&*Loc);
+ // 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;
+ }
+
+ // 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 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) &&
@@ -2111,6 +2148,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
++FI;
}
}
+
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..717bfc28b4065 100644
--- a/llvm/test/DebugInfo/X86/branch-folder-dbg.mir
+++ b/llvm/test/DebugInfo/X86/branch-folder-dbg.mir
@@ -1,16 +1,23 @@
# 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 DBG_PHIs are deleted rather than hoisted.
# 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 +80,8 @@
...
---
name: g
+tracksRegLiveness: true
+isSSA: false
body: |
bb.0 (%ir-block.0):
successors: %bb.1(0x40000000), %bb.2(0x40000000)
@@ -87,21 +96,23 @@ 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_PHI $esp, 3
+ 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_PHI $esp, 4
+ 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
``````````
</details>
https://github.com/llvm/llvm-project/pull/149999
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