Returning poison from a function w/ noundef return attribute is UB

This does for readability of returns within said function as what we do for the caller side when reasoning about what might be poison.

Differential Revision: https://reviews.llvm.org/D111180

Author: preames

llvm/lib/Analysis/ValueTracking.cpp

@@ -5418,7 +5418,10 @@ void llvm::getGuaranteedWellDefinedOps(
       }
       break;
     }
-
+    case Instruction::Ret:
+      if (I->getFunction()->hasRetAttribute(Attribute::NoUndef))
+        Operands.insert(I->getOperand(0));
+      break;
     default:
       break;
   }

llvm/test/Analysis/ScalarEvolution/flags-from-poison.ll

@@ -1621,49 +1621,37 @@ cont6:                                            ; preds = %cont1, %if.then
   br label %for.cond
 }
 
-; TODO: once D111180 lands, remove the udiv from these *-basic tests.
-; noundef really should be enough
-
 define noundef i32 @add-basic(i32 %a, i32 %b) {
 ; CHECK-LABEL: 'add-basic'
 ; CHECK-NEXT:  Classifying expressions for: @add-basic
 ; CHECK-NEXT:    %res = add nuw nsw i32 %a, %b
 ; CHECK-NEXT:    --> (%a + %b)<nuw><nsw> U: full-set S: full-set
-; CHECK-NEXT:    %res2 = udiv i32 255, %res
-; CHECK-NEXT:    --> (255 /u (%a + %b)<nuw><nsw>) U: [0,256) S: [0,256)
 ; CHECK-NEXT:  Determining loop execution counts for: @add-basic
 ;
   %res = add nuw nsw i32 %a, %b
-  %res2 = udiv i32 255, %res
-  ret i32 %res2
+  ret i32 %res
 }
 
 define noundef i32 @sub-basic(i32 %a, i32 %b) {
 ; CHECK-LABEL: 'sub-basic'
 ; CHECK-NEXT:  Classifying expressions for: @sub-basic
 ; CHECK-NEXT:    %res = sub nuw nsw i32 %a, %b
 ; CHECK-NEXT:    --> ((-1 * %b) + %a) U: full-set S: full-set
-; CHECK-NEXT:    %res2 = udiv i32 255, %res
-; CHECK-NEXT:    --> (255 /u ((-1 * %b) + %a)) U: [0,256) S: [0,256)
 ; CHECK-NEXT:  Determining loop execution counts for: @sub-basic
 ;
   %res = sub nuw nsw i32 %a, %b
-  %res2 = udiv i32 255, %res
-  ret i32 %res2
+  ret i32 %res
 }
 
 define noundef i32 @mul-basic(i32 %a, i32 %b) {
 ; CHECK-LABEL: 'mul-basic'
 ; CHECK-NEXT:  Classifying expressions for: @mul-basic
 ; CHECK-NEXT:    %res = mul nuw nsw i32 %a, %b
 ; CHECK-NEXT:    --> (%a * %b)<nuw><nsw> U: full-set S: full-set
-; CHECK-NEXT:    %res2 = udiv i32 255, %res
-; CHECK-NEXT:    --> (255 /u (%a * %b)<nuw><nsw>) U: [0,256) S: [0,256)
 ; CHECK-NEXT:  Determining loop execution counts for: @mul-basic
 ;
   %res = mul nuw nsw i32 %a, %b
-  %res2 = udiv i32 255, %res
-  ret i32 %res2
+  ret i32 %res
 }
 
 @gA = external global i32
@@ -1746,9 +1734,9 @@ define noundef i32 @mul-recurse() {
 ; CHECK-NEXT:    %d = load i32, i32* @gD, align 4
 ; CHECK-NEXT:    --> %d U: full-set S: full-set
 ; CHECK-NEXT:    %x = mul nuw i32 %a, %b
-; CHECK-NEXT:    --> (%a * %b) U: full-set S: full-set
+; CHECK-NEXT:    --> (%a * %b)<nuw> U: full-set S: full-set
 ; CHECK-NEXT:    %y = mul nuw i32 %c, %d
-; CHECK-NEXT:    --> (%c * %d) U: full-set S: full-set
+; CHECK-NEXT:    --> (%c * %d)<nuw> U: full-set S: full-set
 ; CHECK-NEXT:    %res = mul nuw i32 %x, %y
 ; CHECK-NEXT:    --> (%a * %b * %c * %d) U: full-set S: full-set
 ; CHECK-NEXT:  Determining loop execution counts for: @mul-recurse