[BranchFolding] Kill common hoisted debug instructions

[OCHyams]

Please ignore the first 2 commits - they're from #140063

---

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.

[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Orlando Cazalet-Hyams (OCHyams)

Changes

Please ignore the first 2 commits - they're from #140063

---

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/140091.diff

3 Files Affected:

• (modified) llvm/lib/CodeGen/BranchFolding.cpp (+63-1)
• (added) llvm/test/DebugInfo/X86/branch-folder-dbg.mir (+117)
• (added) obj ()

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 6f5afbd2a996a..329c46de5e450 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"
@@ -2071,7 +2072,68 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
   if (!HasDups)
     return false;
 
-  MBB->splice(Loc, TBB, TBB->begin(), TIB);
+  // Hoist the instructions from [T.begin, TIB) and then delete [F.begin, FIB).
+  // 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))) {
+
+      // 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;
+      }
+
+      assert(TI->isIdenticalTo(*FI, MachineInstr::CheckKillDead) &&
+             "Expected non-debug lockstep");
+
+      // Merge debug locs on hoisted instructions.
+      TI->setDebugLoc(
+          DILocation::getMergedLocation(TI->getDebugLoc(), FI->getDebugLoc()));
+      TI->moveBefore(&*Loc);
+      ++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
new file mode 100644
index 0000000000000..831b263fdccd4
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/branch-folder-dbg.mir
@@ -0,0 +1,117 @@
+# 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. 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 (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
+
+--- |
+  target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+  target triple = "x86_64-unknown-linux-gnu"
+
+  declare dso_local noundef i64 @f() local_unnamed_addr
+
+  define dso_local noundef i32 @g() local_unnamed_addr !dbg !7 {
+    %call = tail call noundef i64 @f()
+    %cmp1 = icmp sgt i64 0, %call
+    %conv2 = trunc i64 0 to i32
+    br i1 %cmp1, label %if.then, label %if.else
+
+  if.then:                                          ; preds = %0
+      #dbg_value(i64 0, !11, !DIExpression(), !13)
+    tail call void @_Z3fooii(i32 noundef %conv2, i32 noundef 0), !dbg !14
+      #dbg_value(i64 1, !11, !DIExpression(), !13)
+    br label %if.end, !dbg !15
+
+  if.else:                                          ; preds = %0
+      #dbg_value(i64 2, !11, !DIExpression(), !13)
+    tail call void @_Z3barii(i32 noundef %conv2, i32 noundef 1), !dbg !16
+      #dbg_value(i64 3, !11, !DIExpression(), !13)
+    br label %if.end, !dbg !17
+
+  if.end:                                           ; preds = %if.else, %if.then
+    ret i32 2
+  }
+
+  declare void @_Z3fooii(i32 noundef, i32 noundef) local_unnamed_addr
+
+  declare void @_Z3barii(i32 noundef, i32 noundef) local_unnamed_addr
+
+  !llvm.module.flags = !{!0, !1}
+  !llvm.ident = !{!2}
+  !llvm.dbg.cu = !{!3}
+  !llvm.debugify = !{!5, !6}
+
+  !0 = !{i32 7, !"Dwarf Version", i32 5}
+  !1 = !{i32 2, !"Debug Info Version", i32 3}
+  !2 = !{!"clang version 21.0.0"}
+  !3 = distinct !DICompileUnit(language: DW_LANG_C, file: !4, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+  !4 = !DIFile(filename: "test.nodbg.ll", directory: "/")
+  !5 = !{i32 15}
+  !6 = !{i32 7}
+  !7 = distinct !DISubprogram(name: "g", linkageName: "g", scope: null, file: !4, line: 1, type: !8, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !3, retainedNodes: !10)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{}
+  !10 = !{!11}
+  !11 = !DILocalVariable(name: "1", scope: !7, file: !4, line: 3, type: !12)
+  !12 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
+  !13 = !DILocation(line: 3, column: 1, scope: !7)
+  !14 = !DILocation(line: 9, column: 1, scope: !7)
+  !15 = !DILocation(line: 10, column: 1, scope: !7)
+  !16 = !DILocation(line: 11, column: 1, scope: !7)
+  !17 = !DILocation(line: 12, column: 1, scope: !7)
+...
+---
+name:            g
+tracksRegLiveness: true
+isSSA: false
+body:             |
+  bb.0 (%ir-block.0):
+    successors: %bb.1(0x40000000), %bb.2(0x40000000)
+
+    frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
+    frame-setup CFI_INSTRUCTION def_cfa_offset 16
+    CALL64pcrel32 @f, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    TEST64rr killed renamable $rax, renamable $rax, implicit-def $eflags
+    JCC_1 %bb.2, 9, implicit killed $eflags
+    JMP_1 %bb.1
+
+  bb.1.if.then:
+    successors: %bb.3(0x80000000)
+    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
+    JMP_1 %bb.3,  debug-location !15
+
+  bb.2.if.else:
+    successors: %bb.3(0x80000000)
+
+    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
+
+  bb.3.if.end:
+    $eax = MOV32ri 2
+    $rcx = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    frame-destroy CFI_INSTRUCTION def_cfa_offset 8
+    RET 0, $eax
+...
diff --git a/obj b/obj
new file mode 100644
index 0000000000000..6fd99f7766296
Binary files /dev/null and b/obj differ

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp -- llvm/lib/CodeGen/BranchFolding.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 329c46de5..65d696c8e 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -2084,9 +2084,8 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
           assert(DI->isDebugInstr() && "Expected a debug instruction");
           if (DI->isDebugRef()) {
             MIRBuilder.setDebugLoc(DI->getDebugLoc());
-            MIRBuilder.buildDirectDbgValue(
-                0, DI->getDebugVariable(),
-                DI->getDebugExpression());
+            MIRBuilder.buildDirectDbgValue(0, DI->getDebugVariable(),
+                                           DI->getDebugExpression());
             return;
           }
 
@@ -2107,7 +2106,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
       // instruction handling code).
       while (FI->isDebugOrPseudoInstr()) {
         assert(FI != FE && "Unexpected end of FBB range");
-        MachineBasicBlock::iterator FINext = std::next(FI) ;
+        MachineBasicBlock::iterator FINext = std::next(FI);
         HoistAndKillDbgInstr(FI, Loc);
         FI = FINext;
       }

[SLTozer]

Use of a GlobalISel class here - the post-ISel choice I think should be MachineInstrBuilder.h