[PATCH] D140892: [Local] Fold branch on undef to unreachable

Tue Jan 3 05:56:12 PST 2023

nikic created this revision.
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Branch on undef is UB, so we should fold it to unreachable.

I've cleaned up badly reduced transform tests that were affected by this, but there's still a lot of codegen tests that need to be updated:

  LLVM :: CodeGen/AArch64/arm64-bitfield-extract.ll
  LLVM :: CodeGen/AArch64/arm64-call-tailcalls.ll
  LLVM :: CodeGen/AArch64/arm64-collect-loh.ll
  LLVM :: CodeGen/AArch64/arm64-shrink-wrapping.ll
  LLVM :: CodeGen/AArch64/arm64-storebytesmerge.ll
  LLVM :: CodeGen/AArch64/madd-combiner.ll
  LLVM :: CodeGen/AArch64/optimize-cond-branch.ll
  LLVM :: CodeGen/AArch64/tail-call-unused-zext.ll
  LLVM :: CodeGen/AArch64/tailcall-ssp-split-debug.ll
  LLVM :: CodeGen/AMDGPU/agpr-copy-no-free-registers.ll
  LLVM :: CodeGen/AMDGPU/cgp-bitfield-extract.ll
  LLVM :: CodeGen/AMDGPU/combine-add-zext-xor.ll
  LLVM :: CodeGen/AMDGPU/dagcomb-shuffle-vecextend-non2.ll
  LLVM :: CodeGen/AMDGPU/early-if-convert.ll
  LLVM :: CodeGen/AMDGPU/extract-subvector-16bit.ll
  LLVM :: CodeGen/AMDGPU/extract-subvector.ll
  LLVM :: CodeGen/AMDGPU/i1-copy-implicit-def.ll
  LLVM :: CodeGen/AMDGPU/i1-copy-phi.ll
  LLVM :: CodeGen/AMDGPU/implicit-def-muse.ll
  LLVM :: CodeGen/AMDGPU/infinite-loop.ll
  LLVM :: CodeGen/AMDGPU/inline-asm.ll
  LLVM :: CodeGen/AMDGPU/ipra-return-address-save-restore.ll
  LLVM :: CodeGen/AMDGPU/mfma-cd-select.ll
  LLVM :: CodeGen/AMDGPU/noclobber-barrier.ll
  LLVM :: CodeGen/AMDGPU/si-annotate-cf-noloop.ll
  LLVM :: CodeGen/AMDGPU/si-annotate-cf-unreachable.ll
  LLVM :: CodeGen/AMDGPU/simplifydemandedbits-recursion.ll
  LLVM :: CodeGen/AMDGPU/sink-image-sample.ll
  LLVM :: CodeGen/AMDGPU/skip-if-dead.ll
  LLVM :: CodeGen/AMDGPU/smrd.ll
  LLVM :: CodeGen/AMDGPU/split-smrd.ll
  LLVM :: CodeGen/AMDGPU/subreg-coalescer-crash.ll
  LLVM :: CodeGen/AMDGPU/switch-default-block-unreachable.ll
  LLVM :: CodeGen/AMDGPU/undefined-subreg-liverange.ll
  LLVM :: CodeGen/AMDGPU/uniform-cfg.ll
  LLVM :: CodeGen/AMDGPU/v1024.ll
  LLVM :: CodeGen/AMDGPU/vgpr-liverange-ir.ll
  LLVM :: CodeGen/ARM/2009-11-02-NegativeLane.ll
  LLVM :: CodeGen/ARM/2011-02-07-AntidepClobber.ll
  LLVM :: CodeGen/ARM/2013-07-29-vector-or-combine.ll
  LLVM :: CodeGen/ARM/analyze-branch-bkpt.ll
  LLVM :: CodeGen/ARM/arm-shrink-wrapping.ll
  LLVM :: CodeGen/ARM/call-tc.ll
  LLVM :: CodeGen/ARM/crash-greedy.ll
  LLVM :: CodeGen/ARM/debug-info-qreg.ll
  LLVM :: CodeGen/ARM/fcmp-xo.ll
  LLVM :: CodeGen/ARM/i1.ll
  LLVM :: CodeGen/ARM/ifcvt-branch-weight-bug.ll
  LLVM :: CodeGen/ARM/ifcvt10.ll
  LLVM :: CodeGen/ARM/machine-sink-multidef.ll
  LLVM :: CodeGen/ARM/reg_sequence.ll
  LLVM :: CodeGen/ARM/v8m.base-jumptable_alignment.ll
  LLVM :: CodeGen/ARM/vector-DAGCombine.ll
  LLVM :: CodeGen/AVR/pre-schedule.ll
  LLVM :: CodeGen/Hexagon/addrmode-keepdeadphis.ll
  LLVM :: CodeGen/Hexagon/always-ext.ll
  LLVM :: CodeGen/Hexagon/autohvx/isel-bitcast-vsplat.ll
  LLVM :: CodeGen/Hexagon/autohvx/isel-const-splat-imm.ll
  LLVM :: CodeGen/Hexagon/autohvx/vector-align-only-phi-use.ll
  LLVM :: CodeGen/Hexagon/avoid-predspill-calleesaved.ll
  LLVM :: CodeGen/Hexagon/bit-bitsplit-regclass.ll
  LLVM :: CodeGen/Hexagon/bit-visit-flowq.ll
  LLVM :: CodeGen/Hexagon/branchfolder-keep-impdef.ll
  LLVM :: CodeGen/Hexagon/callR_noreturn.ll
  LLVM :: CodeGen/Hexagon/cfgopt-fall-through.ll
  LLVM :: CodeGen/Hexagon/early-if-vecpred.ll
  LLVM :: CodeGen/Hexagon/frame-offset-overflow.ll
  LLVM :: CodeGen/Hexagon/hvx-bitcast-v64i1.ll
  LLVM :: CodeGen/Hexagon/ignore-terminal-mbb.ll
  LLVM :: CodeGen/Hexagon/isel-setcc-i1.ll
  LLVM :: CodeGen/Hexagon/isel-simplify-crash.ll
  LLVM :: CodeGen/Hexagon/newvalueSameReg.ll
  LLVM :: CodeGen/Hexagon/newvaluejump-kill.ll
  LLVM :: CodeGen/Hexagon/packetize-impdef-1.ll
  LLVM :: CodeGen/Hexagon/rdf-cover-use.ll
  LLVM :: CodeGen/Hexagon/rdf-extra-livein.ll
  LLVM :: CodeGen/Hexagon/rdf-ignore-undef.ll
  LLVM :: CodeGen/Hexagon/rdf-multiple-phis-up.ll
  LLVM :: CodeGen/Hexagon/rdf-phi-shadows.ll
  LLVM :: CodeGen/Hexagon/redundant-branching2.ll
  LLVM :: CodeGen/Hexagon/reg-scavengebug-2.ll
  LLVM :: CodeGen/Hexagon/stack-align-reset.ll
  LLVM :: CodeGen/Hexagon/store-vector-pred.ll
  LLVM :: CodeGen/Hexagon/swp-carried-1.ll
  LLVM :: CodeGen/Hexagon/swp-dep-neg-offset.ll
  LLVM :: CodeGen/Hexagon/swp-epilog-phi12.ll
  LLVM :: CodeGen/Hexagon/swp-epilog-phi13.ll
  LLVM :: CodeGen/Hexagon/swp-epilog-phi9.ll
  LLVM :: CodeGen/Hexagon/swp-maxstart.ll
  LLVM :: CodeGen/Hexagon/swp-phi-dep.ll
  LLVM :: CodeGen/Hexagon/swp-phi-start.ll
  LLVM :: CodeGen/Hexagon/swp-prolog-phi.ll
  LLVM :: CodeGen/Hexagon/swp-tfri.ll
  LLVM :: CodeGen/Hexagon/tail-dup-subreg-map.ll
  LLVM :: CodeGen/Hexagon/v6-unaligned-spill.ll
  LLVM :: CodeGen/Hexagon/v60-cur.ll
  LLVM :: CodeGen/Hexagon/vassign-to-combine.ll
  LLVM :: CodeGen/Hexagon/vect/vsplat-v8i8.ll
  LLVM :: CodeGen/Hexagon/vect/zext-v4i1.ll
  LLVM :: CodeGen/MIR/X86/unreachable_block.ll
  LLVM :: CodeGen/MSP430/umulo-16.ll
  LLVM :: CodeGen/Mips/prevent-hoisting.ll
  LLVM :: CodeGen/PowerPC/aantidep-inline-asm-use.ll
  LLVM :: CodeGen/PowerPC/aix-vsx-splatimm.ll
  LLVM :: CodeGen/PowerPC/bdzlr.ll
  LLVM :: CodeGen/PowerPC/build-vector-to-ld-chain.ll
  LLVM :: CodeGen/PowerPC/ctr-cleanup.ll
  LLVM :: CodeGen/PowerPC/ctrloop-large-ec.ll
  LLVM :: CodeGen/PowerPC/early-ret2.ll
  LLVM :: CodeGen/PowerPC/expand-foldable-isel.ll
  LLVM :: CodeGen/PowerPC/extra-toc-reg-deps.ll
  LLVM :: CodeGen/PowerPC/ifcvt-forked-bug-2016-08-08.ll
  LLVM :: CodeGen/PowerPC/lbzux.ll
  LLVM :: CodeGen/PowerPC/licm-remat.ll
  LLVM :: CodeGen/PowerPC/mma-phi-accs.ll
  LLVM :: CodeGen/PowerPC/negctr.ll
  LLVM :: CodeGen/PowerPC/optimize-andiso.ll
  LLVM :: CodeGen/PowerPC/p10-spill-crun.ll
  LLVM :: CodeGen/PowerPC/post-ra-ec.ll
  LLVM :: CodeGen/PowerPC/ppc-shrink-wrapping.ll
  LLVM :: CodeGen/PowerPC/pr24636.ll
  LLVM :: CodeGen/PowerPC/pr25157-peephole.ll
  LLVM :: CodeGen/PowerPC/pr25157.ll
  LLVM :: CodeGen/PowerPC/pr43527.ll
  LLVM :: CodeGen/PowerPC/pr48519.ll
  LLVM :: CodeGen/PowerPC/pr49509.ll
  LLVM :: CodeGen/PowerPC/redundant-copy-after-tail-dup.ll
  LLVM :: CodeGen/PowerPC/rlwimi-and.ll
  LLVM :: CodeGen/PowerPC/sdag-ppcf128.ll
  LLVM :: CodeGen/PowerPC/sink-side-effect.ll
  LLVM :: CodeGen/PowerPC/sms-iterator.ll
  LLVM :: CodeGen/PowerPC/spill_p9_setb.ll
  LLVM :: CodeGen/PowerPC/stwu-sched.ll
  LLVM :: CodeGen/PowerPC/stwux.ll
  LLVM :: CodeGen/PowerPC/subreg-postra.ll
  LLVM :: CodeGen/PowerPC/test-and-cmp-folding.ll
  LLVM :: CodeGen/PowerPC/vsx-fma-mutate-trivial-copy.ll
  LLVM :: CodeGen/PowerPC/vsx-fma-mutate-undef.ll
  LLVM :: CodeGen/PowerPC/vsx-self-copy.ll
  LLVM :: CodeGen/PowerPC/vsx-spill-norwstore.ll
  LLVM :: CodeGen/PowerPC/xray-ret-is-terminator.ll
  LLVM :: CodeGen/PowerPC/xvcmpeqdp-v2f64.ll
  LLVM :: CodeGen/SPARC/missinglabel.ll
  LLVM :: CodeGen/SystemZ/codegenprepare-sink-and-for-tm.ll
  LLVM :: CodeGen/SystemZ/elim-frame-index-VR.ll
  LLVM :: CodeGen/SystemZ/ipra.ll
  LLVM :: CodeGen/SystemZ/loop-03.ll
  LLVM :: CodeGen/SystemZ/signbits.ll
  LLVM :: CodeGen/SystemZ/stackmap.ll
  LLVM :: CodeGen/SystemZ/store-replicated-vals.ll
  LLVM :: CodeGen/SystemZ/subregliveness-03.ll
  LLVM :: CodeGen/SystemZ/subregliveness-05.ll
  LLVM :: CodeGen/Thumb/consthoist-imm8-costs-1.ll
  LLVM :: CodeGen/Thumb2/2009-08-01-WrongLDRBOpc.ll
  LLVM :: CodeGen/Thumb2/2009-08-06-SpDecBug.ll
  LLVM :: CodeGen/Thumb2/2010-06-21-TailMergeBug.ll
  LLVM :: CodeGen/Thumb2/2010-11-22-EpilogueBug.ll
  LLVM :: CodeGen/Thumb2/2011-06-07-TwoAddrEarlyClobber.ll
  LLVM :: CodeGen/Thumb2/ifcvt-rescan-bug-2016-08-22.ll
  LLVM :: CodeGen/Thumb2/machine-licm.ll
  LLVM :: CodeGen/Thumb2/mve-phireg.ll
  LLVM :: CodeGen/Thumb2/pr52817.ll
  LLVM :: CodeGen/Thumb2/thumb2-ifcvt2.ll
  LLVM :: CodeGen/Thumb2/v8_IT_5.ll
  LLVM :: CodeGen/WebAssembly/cfg-stackify-eh.ll
  LLVM :: CodeGen/WebAssembly/cfg-stackify.ll
  LLVM :: CodeGen/WebAssembly/function-addr-offset.ll
  LLVM :: CodeGen/WebAssembly/implicit-def.ll
  LLVM :: CodeGen/X86/2011-02-21-VirtRegRewriter-KillSubReg.ll
  LLVM :: CodeGen/X86/2011-04-13-SchedCmpJmp.ll
  LLVM :: CodeGen/X86/2011-09-14-valcoalesce.ll
  LLVM :: CodeGen/X86/2012-07-15-broadcastfold.ll
  LLVM :: CodeGen/X86/AMX/amx-ldtilecfg-insert.ll
  LLVM :: CodeGen/X86/AMX/amx-lower-tile-copy.ll
  LLVM :: CodeGen/X86/AMX/amx-spill-merge.ll
  LLVM :: CodeGen/X86/StackColoring.ll
  LLVM :: CodeGen/X86/atomic-bit-test.ll
  LLVM :: CodeGen/X86/avx-cmp.ll
  LLVM :: CodeGen/X86/avx-load-store.ll
  LLVM :: CodeGen/X86/avx-splat.ll
  LLVM :: CodeGen/X86/avx2-vbroadcast.ll
  LLVM :: CodeGen/X86/avx512-i1test.ll
  LLVM :: CodeGen/X86/block-placement.ll
  LLVM :: CodeGen/X86/callbr-asm-blockplacement.ll
  LLVM :: CodeGen/X86/coalesce-esp.ll
  LLVM :: CodeGen/X86/codegen-prepare-cast.ll
  LLVM :: CodeGen/X86/hoist-spill.ll
  LLVM :: CodeGen/X86/jump_sign.ll
  LLVM :: CodeGen/X86/licm-symbol.ll
  LLVM :: CodeGen/X86/machine-cse.ll
  LLVM :: CodeGen/X86/machine-trace-metrics-crash.ll
  LLVM :: CodeGen/X86/mbp-false-cfg-break.ll
  LLVM :: CodeGen/X86/merge-vector-stores-scale-idx-crash.ll
  LLVM :: CodeGen/X86/misched-new.ll
  LLVM :: CodeGen/X86/postra-licm.ll
  LLVM :: CodeGen/X86/pr38795.ll
  LLVM :: CodeGen/X86/pr49451.ll
  LLVM :: CodeGen/X86/pr50254.ll
  LLVM :: CodeGen/X86/pr57673.ll
  LLVM :: CodeGen/X86/ragreedy-hoist-spill.ll
  LLVM :: CodeGen/X86/regalloc-spill-at-ehpad.ll
  LLVM :: CodeGen/X86/shift-combine.ll
  LLVM :: CodeGen/X86/shuffle-combine-crash.ll
  LLVM :: CodeGen/X86/stackmap.ll
  LLVM :: CodeGen/X86/statepoint-cmp-sunk-past-statepoint.ll
  LLVM :: CodeGen/X86/statepoint-ra.ll
  LLVM :: CodeGen/X86/swifterror.ll
  LLVM :: CodeGen/X86/switch.ll
  LLVM :: CodeGen/X86/tail-dup-merge-loop-headers.ll
  LLVM :: CodeGen/X86/tail-merge-unreachable.ll
  LLVM :: CodeGen/X86/tailcall-cgp-dup.ll
  LLVM :: CodeGen/X86/taildup-crash.ll
  LLVM :: CodeGen/X86/vaargs-prolog-insert.ll
  LLVM :: CodeGen/X86/vector-shuffle-combining-avx512bwvl.ll
  LLVM :: CodeGen/X86/x86-shrink-wrapping.ll
  LLVM :: CodeGen/X86/zext-extract_subreg.ll
  LLVM :: DebugInfo/X86/dbg-value-terminator.ll


https://reviews.llvm.org/D140892

Files:
  llvm/lib/Transforms/Utils/Local.cpp
  llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll


Index: llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll
===================================================================

--- llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll
+++ llvm/test/Transforms/SimplifyCFG/branch-on-undef.ll
@@ -6,15 +6,7 @@
 define void @br_undef_simple() {
 ; CHECK-LABEL: @br_undef_simple(
 ; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br i1 undef, label [[IF:%.*]], label [[ELSE:%.*]]
-; CHECK:       common.ret:
-; CHECK-NEXT:    ret void
-; CHECK:       if:
-; CHECK-NEXT:    call void @foo(i32 1)
-; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
-; CHECK:       else:
-; CHECK-NEXT:    call void @foo(i32 2)
-; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK-NEXT:    unreachable
 ;
   call void @foo(i32 0)
   br i1 undef, label %if, label %else
@@ -31,15 +23,7 @@
 define void @br_poison_simple() {
 ; CHECK-LABEL: @br_poison_simple(
 ; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br i1 poison, label [[IF:%.*]], label [[ELSE:%.*]]
-; CHECK:       common.ret:
-; CHECK-NEXT:    ret void
-; CHECK:       if:
-; CHECK-NEXT:    call void @foo(i32 1)
-; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
-; CHECK:       else:
-; CHECK-NEXT:    call void @foo(i32 2)
-; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK-NEXT:    unreachable
 ;
   call void @foo(i32 0)
   br i1 poison, label %if, label %else
@@ -62,16 +46,14 @@
 ; CHECK-NEXT:    ]
 ; CHECK:       br:
 ; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br i1 poison, label [[IF]], label [[ELSE]]
+; CHECK-NEXT:    unreachable
 ; CHECK:       common.ret:
 ; CHECK-NEXT:    ret void
 ; CHECK:       if:
-; CHECK-NEXT:    [[PHI1:%.*]] = phi i32 [ 1, [[BR]] ], [ 2, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    call void @foo(i32 [[PHI1]])
+; CHECK-NEXT:    call void @foo(i32 2)
 ; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
 ; CHECK:       else:
-; CHECK-NEXT:    [[PHI2:%.*]] = phi i32 [ 3, [[BR]] ], [ 4, [[ENTRY]] ]
-; CHECK-NEXT:    call void @foo(i32 [[PHI2]])
+; CHECK-NEXT:    call void @foo(i32 4)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
 ;
 entry:
Index: llvm/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/lib/Transforms/Utils/Local.cpp
+++ llvm/lib/Transforms/Utils/Local.cpp
@@ -181,6 +181,12 @@
       return true;
     }
 
+    if (isa<UndefValue>(BI->getCondition())) {
+      // Branch on undef is undefined behavior.
+      changeToUnreachable(BI, /*PreserveLCSSA*/ false, DTU);
+      return true;
+    }
+
     return false;
   }
 


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