[llvm] b22910d - [InstCombine] erase instructions leading up to unreachable

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2020-09-07T10:44:08-04:00
New Revision: b22910daab95be1ebc6ab8a74190e38130b0e6ef

URL: https://github.com/llvm/llvm-project/commit/b22910daab95be1ebc6ab8a74190e38130b0e6ef
DIFF: https://github.com/llvm/llvm-project/commit/b22910daab95be1ebc6ab8a74190e38130b0e6ef.diff

LOG: [InstCombine] erase instructions leading up to unreachable

Normal dead code elimination ignores assume intrinsics, so we fail to
delete assumes that are not meaningful (and potentially worse if they
cause conflicts with other assumptions).

The motivating example in https://llvm.org/PR47416 suggests that we
might have problems upstream from here (difference between C and C++),
but this should be a cheap way to make sure we remove more dead code.

Differential Revision: https://reviews.llvm.org/D87149

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineInternal.h
    llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    llvm/test/Transforms/InstCombine/assume.ll
    llvm/test/Transforms/InstCombine/pr33689_same_bitwidth.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index a03cb5e47051..62ee7d00780e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -159,6 +159,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
   Instruction *visitFenceInst(FenceInst &FI);
   Instruction *visitSwitchInst(SwitchInst &SI);
   Instruction *visitReturnInst(ReturnInst &RI);
+  Instruction *visitUnreachableInst(UnreachableInst &I);
   Instruction *
   foldAggregateConstructionIntoAggregateReuse(InsertValueInst &OrigIVI);
   Instruction *visitInsertValueInst(InsertValueInst &IV);

diff  --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 178e9a4a17bd..0ca256860c59 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2798,6 +2798,19 @@ Instruction *InstCombinerImpl::visitReturnInst(ReturnInst &RI) {
   return nullptr;
 }
 
+Instruction *InstCombinerImpl::visitUnreachableInst(UnreachableInst &I) {
+  // Try to remove the previous instruction if it must lead to unreachable.
+  // This includes instructions like stores and "llvm.assume" that may not get
+  // removed by simple dead code elimination.
+  Instruction *Prev = I.getPrevNonDebugInstruction();
+  if (Prev && !Prev->isEHPad() &&
+      isGuaranteedToTransferExecutionToSuccessor(Prev)) {
+    eraseInstFromFunction(*Prev);
+    return &I;
+  }
+  return nullptr;
+}
+
 Instruction *InstCombinerImpl::visitUnconditionalBranchInst(BranchInst &BI) {
   assert(BI.isUnconditional() && "Only for unconditional branches.");
 

diff  --git a/llvm/test/Transforms/InstCombine/assume.ll b/llvm/test/Transforms/InstCombine/assume.ll
index b55b1c21c0b9..8ca24caa2aa1 100644
--- a/llvm/test/Transforms/InstCombine/assume.ll
+++ b/llvm/test/Transforms/InstCombine/assume.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -instcombine -S | FileCheck %s
+; RUN: opt < %s -instcombine -S -instcombine-infinite-loop-threshold=2 | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
@@ -543,7 +543,6 @@ define i8 @conflicting_assumptions(i8 %x){
 
 define void @PR36270(i32 %b) {
 ; CHECK-LABEL: @PR36270(
-; CHECK-NEXT:    tail call void @llvm.assume(i1 false)
 ; CHECK-NEXT:    unreachable
 ;
   %B7 = xor i32 -1, 2147483647
@@ -573,8 +572,6 @@ define i32 @unreachable_assume(i32 %x, i32 %y) {
 ; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP3]])
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[CMP4:%.*]] = icmp eq i32 [[X]], 2
-; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP4]])
 ; CHECK-NEXT:    unreachable
 ;
 entry:
@@ -612,11 +609,6 @@ define i32 @unreachable_assumes_and_store(i32 %x, i32 %y, i32* %p) {
 ; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP3]])
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[CMP4:%.*]] = icmp eq i32 [[X]], 2
-; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP4]])
-; CHECK-NEXT:    [[CMP5:%.*]] = icmp ugt i32 [[Y]], 42
-; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP5]])
-; CHECK-NEXT:    store i32 [[X]], i32* [[P:%.*]], align 4
 ; CHECK-NEXT:    unreachable
 ;
 entry:

diff  --git a/llvm/test/Transforms/InstCombine/pr33689_same_bitwidth.ll b/llvm/test/Transforms/InstCombine/pr33689_same_bitwidth.ll
index 0ffd41d60a4c..d533703ac6fb 100644
--- a/llvm/test/Transforms/InstCombine/pr33689_same_bitwidth.ll
+++ b/llvm/test/Transforms/InstCombine/pr33689_same_bitwidth.ll
@@ -17,8 +17,6 @@ define void @f(i1 %cond) {
 ; CHECK-NEXT:    [[T12_SUB:%.*]] = getelementptr inbounds [2 x i32], [2 x i32]* [[T12]], i16 0, i16 0
 ; CHECK-NEXT:    br i1 [[COND:%.*]], label [[BB1:%.*]], label [[BB2:%.*]]
 ; CHECK:       bb1:
-; CHECK-NEXT:    [[T8:%.*]] = ptrtoint [2 x i32]* [[T12]] to i16
-; CHECK-NEXT:    store i16 [[T8]], i16* @a, align 2
 ; CHECK-NEXT:    unreachable
 ; CHECK:       bb2:
 ; CHECK-NEXT:    [[T9:%.*]] = load i16*, i16** @b, align 2