Reland "[SimplifyCFG] Improve the precision of PtrValueMayBeModified"

This relands commit f890f01.

The result value of `getelementptr inbounds (TY, null, not zero)` is a poison value.
We can think of it as undefined behavior.

Author: dianqk

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -7395,7 +7395,16 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I, bool PtrValu
     // Look through GEPs. A load from a GEP derived from NULL is still undefined
     if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
       if (GEP->getPointerOperand() == I) {
-        if (!GEP->isInBounds() || !GEP->hasAllZeroIndices())
+        // The current base address is null, there are four cases to consider:
+        // getelementptr (TY, null, 0)                 -> null
+        // getelementptr (TY, null, not zero)          -> may be modified
+        // getelementptr inbounds (TY, null, 0)        -> null
+        // getelementptr inbounds (TY, null, not zero) -> poison iff null is
+        // undefined?
+        if (!GEP->hasAllZeroIndices() &&
+            (!GEP->isInBounds() ||
+             NullPointerIsDefined(GEP->getFunction(),
+                                  GEP->getPointerAddressSpace())))
           PtrValueMayBeModified = true;
         return passingValueIsAlwaysUndefined(V, GEP, PtrValueMayBeModified);
       }

llvm/test/Transforms/SimplifyCFG/UnreachableEliminate.ll

@@ -444,6 +444,28 @@ else:
 define void @test9_gep_inbounds_nonzero(i1 %X, ptr %Y) {
 ; CHECK-LABEL: @test9_gep_inbounds_nonzero(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = xor i1 [[X:%.*]], true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[Y:%.*]], i64 12
+; CHECK-NEXT:    [[TMP1:%.*]] = call ptr @fn_nonnull_noundef_arg(ptr [[GEP]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %X, label %if, label %else
+
+if:
+  br label %else
+
+else:
+  %phi = phi ptr [ %Y, %entry ], [ null, %if ]
+  %gep = getelementptr inbounds i8, ptr %phi, i64 12
+  call ptr @fn_nonnull_noundef_arg(ptr %gep)
+  ret void
+}
+
+define void @test9_gep_inbounds_nonzero_null_defined(i1 %X, ptr %Y) #0 {
+; CHECK-LABEL: @test9_gep_inbounds_nonzero_null_defined(
+; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[X:%.*]], ptr null, ptr [[Y:%.*]]
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[SPEC_SELECT]], i64 12
 ; CHECK-NEXT:    [[TMP0:%.*]] = call ptr @fn_nonnull_noundef_arg(ptr [[GEP]])
@@ -462,9 +484,30 @@ else:
   ret void
 }
 
+define void @test9_gep_inbounds_unknown_null(i1 %X, ptr %Y, i64 %I) {
+; CHECK-LABEL: @test9_gep_inbounds_unknown_null(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = xor i1 [[X:%.*]], true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[Y:%.*]], i64 [[I:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call ptr @fn_nonnull_noundef_arg(ptr [[GEP]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %X, label %if, label %else
+
+if:
+  br label %else
+
+else:
+  %phi = phi ptr [ %Y, %entry ], [ null, %if ]
+  %gep = getelementptr inbounds i8, ptr %phi, i64 %I
+  call ptr @fn_nonnull_noundef_arg(ptr %gep)
+  ret void
+}
 
-define void @test9_gep_inbouds_unknown_null(i1 %X, ptr %Y, i64 %I) {
-; CHECK-LABEL: @test9_gep_inbouds_unknown_null(
+define void @test9_gep_inbounds_unknown_null_defined(i1 %X, ptr %Y, i64 %I) #0 {
+; CHECK-LABEL: @test9_gep_inbounds_unknown_null_defined(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[X:%.*]], ptr null, ptr [[Y:%.*]]
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[SPEC_SELECT]], i64 [[I:%.*]]
@@ -484,6 +527,27 @@ else:
   ret void
 }
 
+define void @test9_gep_inbounds_unknown_null_call_noundef(i1 %X, ptr %Y, i64 %I) {
+; CHECK-LABEL: @test9_gep_inbounds_unknown_null_call_noundef(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[X:%.*]], ptr null, ptr [[Y:%.*]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr [[SPEC_SELECT]], i64 [[I:%.*]]
+; CHECK-NEXT:    [[TMP0:%.*]] = call ptr @fn_noundef_arg(ptr [[GEP]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %X, label %if, label %else
+
+if:
+  br label %else
+
+else:
+  %phi = phi ptr [ %Y, %entry ], [ null, %if ]
+  %gep = getelementptr inbounds i8, ptr %phi, i64 %I
+  call ptr @fn_noundef_arg(ptr %gep)
+  ret void
+}
+
 define void @test9_gep_unknown_null(i1 %X, ptr %Y, i64 %I) {
 ; CHECK-LABEL: @test9_gep_unknown_null(
 ; CHECK-NEXT:  entry:

llvm/test/Transforms/SimplifyCFG/unreachable-eliminate-on-ret.ll

@@ -55,7 +55,26 @@ define nonnull noundef ptr @test_ret_ptr_nonnull_noundef_gep_nonzero(i1 %cond, p
 ; CHECK-LABEL: @test_ret_ptr_nonnull_noundef_gep_nonzero(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[COND:%.*]], ptr [[X:%.*]], ptr null
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds ptr, ptr [[SPEC_SELECT]], i64 12
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr ptr, ptr [[SPEC_SELECT]], i64 12
+; CHECK-NEXT:    ret ptr [[GEP]]
+;
+entry:
+  br i1 %cond, label %bb1, label %bb2
+
+bb1:
+  br label %bb2
+
+bb2:
+  %phi = phi ptr [ null, %entry ], [ %x, %bb1 ]
+  %gep = getelementptr ptr, ptr %phi, i64 12
+  ret ptr %gep
+}
+
+define nonnull noundef ptr @test_ret_ptr_nonnull_noundef_gep_inbounds_nonzero(i1 %cond, ptr %x) {
+; CHECK-LABEL: @test_ret_ptr_nonnull_noundef_gep_inbounds_nonzero(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COND:%.*]])
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds ptr, ptr [[X:%.*]], i64 12
 ; CHECK-NEXT:    ret ptr [[GEP]]
 ;
 entry: