RegisterCoalescer: Don't delete IMPLICIT_DEF if it's live into the same block

[arsenm]

Summary

Live out implicit_defs need to be kept, but the check for this only
checked if the block parent was the same. This doesn't work if the
parent blocks are the same but the value is live. Fixes verifier error
"Instruction ending live segment doesn't read the register", which
would appear at the coalesced non-implicit_def def.

Fixes #38788

[uabelho]

I've done quite some testing on our downstream target with this and
https://reviews.llvm.org/D158850
without problems so it's probably not totally wrong :)

I'll leave for someone who knows the code to accept it.

llvm/test/CodeGen/X86/pr38795-verifier-error-pr38788.ll

1

I don't know what file this is? It doesn't exist in the repo and it's not added in
https://reviews.llvm.org/D158850

Something you only have in your local repo?

[arsenm]

llvm/test/CodeGen/X86/pr38795-verifier-error-pr38788.ll

1

I just moved this to the main test file

[bjope]

llvm/test/CodeGen/X86/pr38795-verifier-error-pr38788.ll

1

The file doesn't exist on main. So when trying to apply this patch it fails since there is no file with this name to delete. So there is something weird with "Diff 1" in the differential.

llvm/test/CodeGen/X86/pr38795.ll

2–3

I guess we want to do this now. I.e. enable verify-machineinstrs.

(PR39440 == GH issue 38788)

[arsenm]

squash test

[bjope]

I'm confused. In "Diff 2" (that now is approved) the things you did in "Diff 1" was removed,
The "diff against base" now seem to show the diff compared to the base of D158850. Which is a not-yet-approved patch prio to this in the patch stack.

So something seems really weird to me. Should this patch perhaps be rebased on top of D158850? And what about the original code changes that were in here before the "squash test" update?

(I still expect that this patch would change the RUN line in llvm/test/CodeGen/X86/pr38795.ll to enable -verify-machineinstrs as part of solving PR39440 == GH issue 38788. Right?)

[arsenm]

Correct diff

[bjope]

llvm/test/CodeGen/X86/pr38795.ll

2–3

Still think you want to remove the FIXME and enabe the verifier, right?
(LGTM otherwise.)

[arsenm]

llvm/test/CodeGen/X86/pr38795.ll

2–3

Only in the next patch, the first one doesn't fix it

[arsenm]

cd4b906e183ffa6b78b2f2d062626ee5f54ceba3

llvm/lib/CodeGen/RegisterCoalescer.cpp

JoinVals::analyzeValue(unsigned ValNo, JoinVals &Other) {
   if (OtherV.ErasableImplicitDef) {
     // Check if OtherV is an IMPLICIT_DEF that extends beyond its basic block.
     // This shouldn't normally happen, but ProcessImplicitDefs can leave such
     // IMPLICIT_DEF instructions behind, and there is nothing wrong with it
     // technically.
     //
     // When it happens, treat that IMPLICIT_DEF as a normal value, and don't try
     // to erase the IMPLICIT_DEF instruction.
+    //
+    // Additionally we must keep an IMPLICIT_DEF if we're redefining an incoming
+    // value.
+
     MachineInstr *OtherImpDef =
         Indexes->getInstructionFromIndex(V.OtherVNI->def);
     MachineBasicBlock *OtherMBB = OtherImpDef->getParent();
-    if (DefMI && DefMI->getParent() != OtherMBB) {
+    if (DefMI &&
+        (DefMI->getParent() != OtherMBB || LIS->isLiveInToMBB(LR, OtherMBB))) {
       LLVM_DEBUG(dbgs() << "IMPLICIT_DEF defined at " << V.OtherVNI->def
                  << " extends into "
                  << printMBBReference(*DefMI->getParent())
                  << ", keeping it.\n");
       OtherV.mustKeepImplicitDef(*TRI, *OtherImpDef);
     } else if (OtherMBB->hasEHPadSuccessor()) {
       // If OtherV is defined in a basic block that has EH pad successors then
       // we get the same problem not just if OtherV is live beyond its basic

llvm/test/CodeGen/X86/pr38795-verifier-error-pr38788.ll

-; RUN: llc -mtriple=i386-unknown-linux-gnu -o - -verify-machineinstrs %s | FileCheck %s
-
-define void @fn(i1 %cmp11) {
-entry:
-  br label %for.cond
-
-for.cond:                                         ; preds = %for.inc, %entry
-  %p.0 = phi i8 [ 0, %entry ], [ %p.2, %for.inc ]
-  %k.0 = phi i8 [ 0, %entry ], [ %k.2, %for.inc ]
-  br i1 %cmp11, label %for.cond35, label %if.end
-
-if.end:                                           ; preds = %for.cond
-  br i1 %cmp11, label %if.then13, label %if.end26
-
-if.then13:                                        ; preds = %if.end
-  br i1 %cmp11, label %for.inc, label %for.cond35
-
-if.end26:                                         ; preds = %for.cond35, %if.end
-  %p.1 = phi i8 [ %p.4, %for.cond35 ], [ %k.0, %if.end ]
-  %k.1 = phi i8 [ %k.4, %for.cond35 ], [ 0, %if.end ]
-  %tobool30 = icmp ne i8 %p.0, 0
-  %spec.select1 = select i1 %tobool30, i8 %k.1, i8 0
-  br label %for.inc
-
-for.inc:                                          ; preds = %if.end26, %if.then13
-  %p.2 = phi i8 [ poison, %if.then13 ], [ %p.1, %if.end26 ]
-  %k.2 = phi i8 [ 0, %if.then13 ], [ %spec.select1, %if.end26 ]
-  %0 = load i32, ptr null, align 4
-  br label %for.cond
-
-for.cond35:                                       ; preds = %if.then13, %for.cond
-  %p.4 = phi i8 [ %k.0, %if.then13 ], [ 0, %for.cond ]
-  %k.4 = phi i8 [ 0, %if.then13 ], [ %k.0, %for.cond ]
-  %tobool36 = icmp eq i32 0, 0
-  br label %if.end26
-}

llvm/test/CodeGen/X86/pr38795-verifier-error-pr38788.mir

bb.8:
     %0:gr32 = COPY killed %4
     JMP_1 %bb.1
 
   bb.9:
     %3:gr32 = COPY killed %2
     JMP_1 %bb.6
 
 ...
+
+# Still failed after patch for first 2 failures
+
+---
+name:            pr38795_verifier_error_reduced_3
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: pr38795_verifier_error_reduced_3
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.5(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:gr32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:gr32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   JCC_1 %bb.5, 4, implicit killed undef $eflags
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.4(0x40000000), %bb.5(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[DEF2:%[0-9]+]]:gr32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gr32 = COPY [[DEF]]
+  ; CHECK-NEXT:   [[MOV32r0_:%[0-9]+]]:gr32 = MOV32r0 implicit-def dead $eflags
+  ; CHECK-NEXT:   JCC_1 %bb.4, 5, implicit killed undef $eflags
+  ; CHECK-NEXT:   JMP_1 %bb.5
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.4(0x40000000), %bb.3(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   TEST32rr undef [[COPY]], [[COPY]], implicit-def $eflags
+  ; CHECK-NEXT:   [[DEF1:%[0-9]+]]:gr32 = IMPLICIT_DEF
+  ; CHECK-NEXT:   JCC_1 %bb.4, 4, implicit killed undef $eflags
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   successors: %bb.4(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:gr32 = COPY [[MOV32r0_]]
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:gr32 = COPY [[COPY1]]
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.5:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   JMP_1 %bb.2
+  bb.0:
+    %0:gr32 = IMPLICIT_DEF
+    %1:gr32 = IMPLICIT_DEF
+    JCC_1 %bb.5, 4, implicit killed undef $eflags
+
+  bb.1:
+    %2:gr32 = IMPLICIT_DEF
+    %1:gr32 = COPY killed %0
+    %0:gr32 = MOV32r0 implicit-def dead $eflags
+    JCC_1 %bb.4, 5, implicit killed undef $eflags
+    JMP_1 %bb.5
+
+  bb.2:
+    TEST32rr undef %1, %1, implicit-def $eflags
+    %2:gr32 = IMPLICIT_DEF
+    JCC_1 %bb.4, 4, implicit killed undef $eflags
+
+  bb.3:
+    %2:gr32 = COPY killed %0
+
+  bb.4:
+    %0:gr32 = COPY killed %2
+    JMP_1 %bb.1
+
+  bb.5:
+    JMP_1 %bb.2
+
+...

llvm/test/CodeGen/X86/pr38795.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; FIXME: Fix machine verifier issues and remove -verify-machineinstrs=0. PR39440.
 ; RUN: llc %s -O2 -mtriple=i386-unknown-linux-gnu -o - -verify-machineinstrs=0 | FileCheck %s
 @.str = external dso_local unnamed_addr constant [6 x i8], align 1
 @a = external dso_local local_unnamed_addr global i32, align 4
 @h = external dso_local local_unnamed_addr global i32, align 4
 @g = external dso_local local_unnamed_addr global i8, align 1
 
 define dso_local void @fn() {
 ; CHECK-LABEL: fn:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    pushl %ebp
 ; CHECK-NEXT:    .cfi_def_cfa_offset 8
 ; CHECK-NEXT:    pushl %ebx
 ; CHECK-NEXT:    .cfi_def_cfa_offset 12
 ; CHECK-NEXT:    pushl %edi
 ; CHECK-NEXT:    .cfi_def_cfa_offset 16
 ; CHECK-NEXT:    pushl %esi
 ; CHECK-NEXT:    .cfi_def_cfa_offset 20
 ; CHECK-NEXT:    subl $28, %esp
 ; CHECK-NEXT:    .cfi_def_cfa_offset 48
 ; CHECK-NEXT:    .cfi_offset %esi, -20
 ; CHECK-NEXT:    .cfi_offset %edi, -16
 ; CHECK-NEXT:    .cfi_offset %ebx, -12
 ; CHECK-NEXT:    .cfi_offset %ebp, -8
 ; CHECK-NEXT:    xorl %ebx, %ebx
-; CHECK-NEXT:    # implicit-def: $ecx
+; CHECK-NEXT:    # implicit-def: $esi
 ; CHECK-NEXT:    # implicit-def: $edi
-; CHECK-NEXT:    # implicit-def: $al
-; CHECK-NEXT:    # kill: killed $al
+; CHECK-NEXT:    # implicit-def: $ch
 ; CHECK-NEXT:    # implicit-def: $dl
 ; CHECK-NEXT:    # implicit-def: $ebp
 ; CHECK-NEXT:    jmp .LBB0_1
 ; CHECK-NEXT:    .p2align 4, 0x90
-; CHECK-NEXT:  .LBB0_16: # %for.inc
-; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    movb %dl, {{[-0-9]+}}(%e{{[sb]}}p) # 1-byte Spill
-; CHECK-NEXT:    movb %dh, %dl
+; CHECK-NEXT:  .LBB0_14: # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    movb %dl, %ch
+; CHECK-NEXT:    movl %ecx, %edx
 ; CHECK-NEXT:  .LBB0_1: # %for.cond
 ; CHECK-NEXT:    # =>This Loop Header: Depth=1
-; CHECK-NEXT:    # Child Loop BB0_20 Depth 2
+; CHECK-NEXT:    # Child Loop BB0_22 Depth 2
 ; CHECK-NEXT:    cmpb $8, %dl
 ; CHECK-NEXT:    movb %dl, {{[-0-9]+}}(%e{{[sb]}}p) # 1-byte Spill
 ; CHECK-NEXT:    ja .LBB0_3
 ; CHECK-NEXT:  # %bb.2: # %for.cond
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    testb %bl, %bl
 ; CHECK-NEXT:    je .LBB0_3
 ; CHECK-NEXT:  # %bb.4: # %if.end
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    movl %ecx, %eax
+; CHECK-NEXT:    movl %esi, %ecx
+; CHECK-NEXT:    movl %esi, %eax
 ; CHECK-NEXT:    cltd
 ; CHECK-NEXT:    idivl a
 ; CHECK-NEXT:    movzbl {{[-0-9]+}}(%e{{[sb]}}p), %edx # 1-byte Folded Reload
-; CHECK-NEXT:    movb %cl, %dh
+; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
 ; CHECK-NEXT:    movl $0, h
 ; CHECK-NEXT:    cmpb $8, %dl
 ; CHECK-NEXT:    jg .LBB0_8
 ; CHECK-NEXT:  # %bb.5: # %if.then13
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    movl %eax, %esi
 ; CHECK-NEXT:    movl $.str, (%esp)
-; CHECK-NEXT:    movb %dh, {{[-0-9]+}}(%e{{[sb]}}p) # 1-byte Spill
+; CHECK-NEXT:    movb %cl, {{[-0-9]+}}(%e{{[sb]}}p) # 1-byte Spill
 ; CHECK-NEXT:    calll printf
-; CHECK-NEXT:    movb {{[-0-9]+}}(%e{{[sb]}}p), %dh # 1-byte Reload
+; CHECK-NEXT:    movzbl {{[-0-9]+}}(%e{{[sb]}}p), %ecx # 1-byte Folded Reload
 ; CHECK-NEXT:    testb %bl, %bl
-; CHECK-NEXT:    movl %esi, %ecx
 ; CHECK-NEXT:    # implicit-def: $eax
-; CHECK-NEXT:    movb {{[-0-9]+}}(%e{{[sb]}}p), %dl # 1-byte Reload
-; CHECK-NEXT:    movb %dl, {{[-0-9]+}}(%e{{[sb]}}p) # 1-byte Spill
-; CHECK-NEXT:    movb %dh, %dl
-; CHECK-NEXT:    jne .LBB0_16
-; CHECK-NEXT:    jmp .LBB0_6
+; CHECK-NEXT:    movb {{[-0-9]+}}(%e{{[sb]}}p), %ch # 1-byte Reload
+; CHECK-NEXT:    movl %ecx, %edx
+; CHECK-NEXT:    je .LBB0_6
+; CHECK-NEXT:    jmp .LBB0_18
 ; CHECK-NEXT:    .p2align 4, 0x90
 ; CHECK-NEXT:  .LBB0_3: # %if.then
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    movl $.str, (%esp)
+; CHECK-NEXT:    movb %ch, {{[-0-9]+}}(%e{{[sb]}}p) # 1-byte Spill
 ; CHECK-NEXT:    calll printf
+; CHECK-NEXT:    movb {{[-0-9]+}}(%e{{[sb]}}p), %ch # 1-byte Reload
 ; CHECK-NEXT:    movzbl {{[-0-9]+}}(%e{{[sb]}}p), %edx # 1-byte Folded Reload
 ; CHECK-NEXT:    # implicit-def: $eax
 ; CHECK-NEXT:  .LBB0_6: # %for.cond35
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    testl %edi, %edi
 ; CHECK-NEXT:    je .LBB0_7
 ; CHECK-NEXT:  .LBB0_11: # %af
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    testb %bl, %bl
 ; CHECK-NEXT:    jne .LBB0_12
-; CHECK-NEXT:  .LBB0_17: # %if.end39
+; CHECK-NEXT:  .LBB0_19: # %if.end39
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    testl %eax, %eax
-; CHECK-NEXT:    je .LBB0_19
-; CHECK-NEXT:  # %bb.18: # %if.then41
+; CHECK-NEXT:    je .LBB0_21
+; CHECK-NEXT:  # %bb.20: # %if.then41
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    movl $0, {{[0-9]+}}(%esp)
 ; CHECK-NEXT:    movl $fn, {{[0-9]+}}(%esp)
 ; CHECK-NEXT:    movl $.str, (%esp)
 ; CHECK-NEXT:    calll printf
-; CHECK-NEXT:  .LBB0_19: # %for.end46
+; CHECK-NEXT:  .LBB0_21: # %for.end46
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    # implicit-def: $dl
-; CHECK-NEXT:    # implicit-def: $dh
+; CHECK-NEXT:    # implicit-def: $ch
+; CHECK-NEXT:    # implicit-def: $cl
 ; CHECK-NEXT:    # implicit-def: $ebp
-; CHECK-NEXT:    jmp .LBB0_20
+; CHECK-NEXT:    jmp .LBB0_22
 ; CHECK-NEXT:    .p2align 4, 0x90
 ; CHECK-NEXT:  .LBB0_8: # %if.end21
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    # implicit-def: $ebp
-; CHECK-NEXT:    jmp .LBB0_9
+; CHECK-NEXT:    testb %bl, %bl
+; CHECK-NEXT:    je .LBB0_13
 ; CHECK-NEXT:    .p2align 4, 0x90
+; CHECK-NEXT:  .LBB0_10: # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    # implicit-def: $eax
+; CHECK-NEXT:    testb %bl, %bl
+; CHECK-NEXT:    je .LBB0_19
+; CHECK-NEXT:  .LBB0_12: # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    # implicit-def: $edi
+; CHECK-NEXT:    # implicit-def: $ch
+; CHECK-NEXT:    # implicit-def: $dl
+; CHECK-NEXT:    # implicit-def: $ebp
+; CHECK-NEXT:    testl %edi, %edi
+; CHECK-NEXT:    jne .LBB0_11
 ; CHECK-NEXT:  .LBB0_7: # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    xorl %edi, %edi
-; CHECK-NEXT:    movb %dl, %dh
-; CHECK-NEXT:    movb {{[-0-9]+}}(%e{{[sb]}}p), %dl # 1-byte Reload
+; CHECK-NEXT:    movb %dl, %cl
 ; CHECK-NEXT:    .p2align 4, 0x90
-; CHECK-NEXT:  .LBB0_20: # %for.cond47
+; CHECK-NEXT:  .LBB0_22: # %for.cond47
 ; CHECK-NEXT:    # Parent Loop BB0_1 Depth=1
 ; CHECK-NEXT:    # => This Inner Loop Header: Depth=2
 ; CHECK-NEXT:    testb %bl, %bl
-; CHECK-NEXT:    jne .LBB0_20
-; CHECK-NEXT:  # %bb.21: # %for.cond47
-; CHECK-NEXT:    # in Loop: Header=BB0_20 Depth=2
+; CHECK-NEXT:    jne .LBB0_22
+; CHECK-NEXT:  # %bb.23: # %for.cond47
+; CHECK-NEXT:    # in Loop: Header=BB0_22 Depth=2
 ; CHECK-NEXT:    testb %bl, %bl
-; CHECK-NEXT:    jne .LBB0_20
-; CHECK-NEXT:  .LBB0_9: # %ae
-; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    jne .LBB0_22
+; CHECK-NEXT:  # %bb.24: # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    movb %ch, %dl
 ; CHECK-NEXT:    testb %bl, %bl
 ; CHECK-NEXT:    jne .LBB0_10
-; CHECK-NEXT:  # %bb.13: # %if.end26
+; CHECK-NEXT:  .LBB0_13: # %if.end26
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    xorl %ecx, %ecx
+; CHECK-NEXT:    xorl %esi, %esi
 ; CHECK-NEXT:    testb %dl, %dl
-; CHECK-NEXT:    je .LBB0_16
-; CHECK-NEXT:  # %bb.14: # %if.end26
+; CHECK-NEXT:    je .LBB0_14
+; CHECK-NEXT:  # %bb.15: # %if.end26
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
 ; CHECK-NEXT:    testl %ebp, %ebp
 ; CHECK-NEXT:    jne .LBB0_16
-; CHECK-NEXT:  # %bb.15: # %if.then31
+; CHECK-NEXT:  # %bb.17: # %if.then31
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    xorl %ecx, %ecx
+; CHECK-NEXT:    xorl %esi, %esi
+; CHECK-NEXT:    movb %dl, %ch
 ; CHECK-NEXT:    xorl %ebp, %ebp
-; CHECK-NEXT:    jmp .LBB0_16
+; CHECK-NEXT:  .LBB0_18: # %for.inc
+; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    movl %ecx, %edx
+; CHECK-NEXT:    jmp .LBB0_1
 ; CHECK-NEXT:    .p2align 4, 0x90
-; CHECK-NEXT:  .LBB0_10: # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    # implicit-def: $eax
-; CHECK-NEXT:    testb %bl, %bl
-; CHECK-NEXT:    je .LBB0_17
-; CHECK-NEXT:  .LBB0_12: # in Loop: Header=BB0_1 Depth=1
-; CHECK-NEXT:    # implicit-def: $edi
-; CHECK-NEXT:    # implicit-def: $cl
-; CHECK-NEXT:    # kill: killed $cl
-; CHECK-NEXT:    # implicit-def: $dl
-; CHECK-NEXT:    # implicit-def: $ebp
-; CHECK-NEXT:    testl %edi, %edi
-; CHECK-NEXT:    jne .LBB0_11
-; CHECK-NEXT:    jmp .LBB0_7
+; CHECK-NEXT:  .LBB0_16: # in Loop: Header=BB0_1 Depth=1
+; CHECK-NEXT:    movb %dl, %ch
+; CHECK-NEXT:    movl %ecx, %edx
+; CHECK-NEXT:    jmp .LBB0_1
 entry:
   br label %for.cond
 
 for.cond:                                         ; preds = %for.inc, %entry
   %l.0 = phi i16 [ undef, %entry ], [ 0, %for.inc ]
   %m.0 = phi i32 [ undef, %entry ], [ %m.2, %for.inc ]
   %i.0 = phi i32 [ undef, %entry ], [ %i.2, %for.inc ]
   %p.0 = phi i8 [ undef, %entry ], [ %p.2, %for.inc ]
for.end46:                                        ; preds = %if.then41, %if.end39, %for.cond35
   br label %for.cond47
 
 for.cond47:                                       ; preds = %for.cond47, %for.end46
   %brmerge = or i1 false, undef
   br i1 %brmerge, label %for.cond47, label %ae
 }
 
 declare dso_local void @printf(ptr nocapture readonly, ...) local_unnamed_addr
+
+
+; Testcase used to fail -verify-coalescing, reduced IR test which
+; failed.
+define void @verifier_error_reduced_issue38788(i1 %cmp11) {
+; CHECK-LABEL: verifier_error_reduced_issue38788:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    pushl %ebx
+; CHECK-NEXT:    .cfi_def_cfa_offset 8
+; CHECK-NEXT:    .cfi_offset %ebx, -8
+; CHECK-NEXT:    movzbl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:    xorl %ecx, %ecx
+; CHECK-NEXT:    xorl %ebx, %ebx
+; CHECK-NEXT:    jmp .LBB1_1
+; CHECK-NEXT:    .p2align 4, 0x90
+; CHECK-NEXT:  .LBB1_7: # %if.end26
+; CHECK-NEXT:    # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    movl %ecx, %edx
+; CHECK-NEXT:  .LBB1_8: # %for.inc
+; CHECK-NEXT:    # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    movl %eax, %ecx
+; CHECK-NEXT:    movl %edx, %ebx
+; CHECK-NEXT:    movzbl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:  .LBB1_1: # %for.cond
+; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    testb $1, %al
+; CHECK-NEXT:    je .LBB1_3
+; CHECK-NEXT:  # %bb.2: # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    jmp .LBB1_5
+; CHECK-NEXT:    .p2align 4, 0x90
+; CHECK-NEXT:  .LBB1_3: # %if.end
+; CHECK-NEXT:    # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    testb $1, %al
+; CHECK-NEXT:    je .LBB1_4
+; CHECK-NEXT:  # %bb.9: # %if.then13
+; CHECK-NEXT:    # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    xorl %edx, %edx
+; CHECK-NEXT:    testb $1, %al
+; CHECK-NEXT:    movl %ebx, %eax
+; CHECK-NEXT:    movl $0, %ebx
+; CHECK-NEXT:    jne .LBB1_8
+; CHECK-NEXT:    jmp .LBB1_5
+; CHECK-NEXT:    .p2align 4, 0x90
+; CHECK-NEXT:  .LBB1_4: # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    movl %ebx, %eax
+; CHECK-NEXT:    xorl %ebx, %ebx
+; CHECK-NEXT:  .LBB1_5: # %if.end26
+; CHECK-NEXT:    # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    testb %cl, %cl
+; CHECK-NEXT:    je .LBB1_7
+; CHECK-NEXT:  # %bb.6: # %if.end26
+; CHECK-NEXT:    # in Loop: Header=BB1_1 Depth=1
+; CHECK-NEXT:    movl %ebx, %ecx
+; CHECK-NEXT:    jmp .LBB1_7
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %p.0 = phi i8 [ 0, %entry ], [ %p.2, %for.inc ]
+  %k.0 = phi i8 [ 0, %entry ], [ %k.2, %for.inc ]
+  br i1 %cmp11, label %for.cond35, label %if.end
+
+if.end:                                           ; preds = %for.cond
+  br i1 %cmp11, label %if.then13, label %if.end26
+
+if.then13:                                        ; preds = %if.end
+  br i1 %cmp11, label %for.inc, label %for.cond35
+
+if.end26:                                         ; preds = %for.cond35, %if.end
+  %p.1 = phi i8 [ %p.4, %for.cond35 ], [ %k.0, %if.end ]
+  %k.1 = phi i8 [ %k.4, %for.cond35 ], [ 0, %if.end ]
+  %tobool30 = icmp ne i8 %p.0, 0
+  %spec.select1 = select i1 %tobool30, i8 %k.1, i8 0
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end26, %if.then13
+  %p.2 = phi i8 [ poison, %if.then13 ], [ %p.1, %if.end26 ]
+  %k.2 = phi i8 [ 0, %if.then13 ], [ %spec.select1, %if.end26 ]
+  %0 = load i32, ptr null, align 4
+  br label %for.cond
+
+for.cond35:                                       ; preds = %if.then13, %for.cond
+  %p.4 = phi i8 [ %k.0, %if.then13 ], [ 0, %for.cond ]
+  %k.4 = phi i8 [ 0, %if.then13 ], [ %k.0, %for.cond ]
+  %tobool36 = icmp eq i32 0, 0
+  br label %if.end26
+}