[InstCombine] Fold assume(false) to non-terminator unreachable

assume(false) is immediate UB, so fold it to (non-terminator)
unreachable.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -2773,6 +2773,12 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     if (Known.isAllOnes() && isAssumeWithEmptyBundle(cast<AssumeInst>(*II)))
       return eraseInstFromFunction(*II);
 
+    // assume(false) is unreachable.
+    if (match(IIOperand, m_CombineOr(m_Zero(), m_Undef()))) {
+      CreateNonTerminatorUnreachable(II);
+      return eraseInstFromFunction(*II);
+    }
+
     // Update the cache of affected values for this assumption (we might be
     // here because we just simplified the condition).
     AC.updateAffectedValues(cast<AssumeInst>(II));

llvm/test/Transforms/InstCombine/assume.ll

@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -passes=instcombine -S  -instcombine-infinite-loop-threshold=3  | FileCheck --check-prefixes=CHECK,DEFAULT %s
-; RUN: opt < %s -passes=instcombine --enable-knowledge-retention -S  -instcombine-infinite-loop-threshold=3  | FileCheck --check-prefixes=CHECK,BUNDLES %s
+; RUN: opt < %s -passes=instcombine -S  -instcombine-infinite-loop-threshold=2  | FileCheck --check-prefixes=CHECK,DEFAULT %s
+; RUN: opt < %s -passes=instcombine --enable-knowledge-retention -S  -instcombine-infinite-loop-threshold=2  | FileCheck --check-prefixes=CHECK,BUNDLES %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
@@ -383,8 +383,8 @@ define i1 @nonnull5(ptr %a) {
 
 define i32 @assumption_conflicts_with_known_bits(i32 %a, i32 %b) {
 ; CHECK-LABEL: @assumption_conflicts_with_known_bits(
-; CHECK-NEXT:    tail call void @llvm.assume(i1 false)
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
+; CHECK-NEXT:    ret i32 poison
 ;
   %and1 = and i32 %b, 3
   %B1 = lshr i32 %and1, %and1
@@ -403,12 +403,8 @@ define i32 @assumption_conflicts_with_known_bits(i32 %a, i32 %b) {
 
 define void @debug_interference(i8 %x) {
 ; CHECK-LABEL: @debug_interference(
-; CHECK-NEXT:    [[CMP2:%.*]] = icmp ne i8 [[X:%.*]], 0
 ; CHECK-NEXT:    tail call void @llvm.dbg.value(metadata i32 5, metadata [[META7:![0-9]+]], metadata !DIExpression()), !dbg [[DBG9:![0-9]+]]
-; CHECK-NEXT:    tail call void @llvm.assume(i1 false)
-; CHECK-NEXT:    tail call void @llvm.dbg.value(metadata i32 5, metadata [[META7]], metadata !DIExpression()), !dbg [[DBG9]]
-; CHECK-NEXT:    tail call void @llvm.dbg.value(metadata i32 5, metadata [[META7]], metadata !DIExpression()), !dbg [[DBG9]]
-; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP2]])
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret void
 ;
   %cmp1 = icmp eq i8 %x, 0
@@ -575,10 +571,8 @@ define void @always_true_assumption() {
 
 define i64 @PR31809() {
 ; CHECK-LABEL: @PR31809(
-; CHECK-NEXT:    [[A:%.*]] = alloca i32, align 4
-; CHECK-NEXT:    [[T1:%.*]] = ptrtoint ptr [[A]] to i64
-; CHECK-NEXT:    call void @llvm.assume(i1 false)
-; CHECK-NEXT:    ret i64 [[T1]]
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
+; CHECK-NEXT:    ret i64 poison
 ;
   %a = alloca i32
   %t1 = ptrtoint ptr %a to i64
@@ -592,10 +586,8 @@ define i64 @PR31809() {
 
 define i8 @conflicting_assumptions(i8 %x){
 ; CHECK-LABEL: @conflicting_assumptions(
-; CHECK-NEXT:    call void @llvm.assume(i1 false)
-; CHECK-NEXT:    [[COND2:%.*]] = icmp eq i8 [[X:%.*]], 4
-; CHECK-NEXT:    call void @llvm.assume(i1 [[COND2]])
-; CHECK-NEXT:    ret i8 5
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
+; CHECK-NEXT:    ret i8 poison
 ;
   %add = add i8 %x, 1
   %cond1 = icmp eq i8 %x, 3

llvm/test/Transforms/InstCombine/out-of-bounds-indexes.ll

@@ -5,9 +5,8 @@
 define i32 @test_out_of_bounds(i32 %a, i1 %x, i1 %y) {
 ; CHECK-LABEL: @test_out_of_bounds(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[AND1:%.*]] = and i32 [[A:%.*]], 3
-; CHECK-NEXT:    tail call void @llvm.assume(i1 poison)
-; CHECK-NEXT:    ret i32 [[AND1]]
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
+; CHECK-NEXT:    ret i32 poison
 ;
 entry:
   %and1 = and i32 %a, 3
@@ -19,9 +18,8 @@ entry:
 
 define i128 @test_non64bit(i128 %a) {
 ; CHECK-LABEL: @test_non64bit(
-; CHECK-NEXT:    [[AND1:%.*]] = and i128 [[A:%.*]], 3
-; CHECK-NEXT:    tail call void @llvm.assume(i1 poison)
-; CHECK-NEXT:    ret i128 [[AND1]]
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
+; CHECK-NEXT:    ret i128 poison
 ;
   %and1 = and i128 %a, 3
   %B = lshr i128 %and1, -1