From b53af0e7f952feb0a54eff2ef9c9e38e62620a4b Mon Sep 17 00:00:00 2001
From: Johannes Doerfert <johannes@jdoerfert.de>
Date: Fri, 14 Feb 2020 20:08:20 -0600
Subject: [PATCH] [Attributor][FIX] Collapse `undef` to a proper value

If we see an undef we cannot assume it's the same as "no value". For now
we just collapse it to 0.
---
 llvm/lib/Transforms/IPO/Attributor.cpp        |  2 +
 llvm/test/Transforms/Attributor/range.ll      | 59 +++++++++++++++++++
 .../Attributor/undefined_behavior.ll          |  5 +-
 3 files changed, 63 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index 0ca65eb55566..1c259694f5ee 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -6656,6 +6656,8 @@ struct AAValueConstantRangeFloating : AAValueConstantRangeImpl {
     }
 
     if (isa<UndefValue>(&V)) {
+      // Collapse the undef state to 0.
+      unionAssumed(ConstantRange(APInt(getBitWidth(), 0)));
       indicateOptimisticFixpoint();
       return;
     }
diff --git a/llvm/test/Transforms/Attributor/range.ll b/llvm/test/Transforms/Attributor/range.ll
index f943f650a02c..6672d800594f 100644
--- a/llvm/test/Transforms/Attributor/range.ll
+++ b/llvm/test/Transforms/Attributor/range.ll
@@ -1214,6 +1214,65 @@ define i1 @fcmp_caller(float %fa, float %fb, double %da, double %db, double* %dp
   ret i1 %o3
 }
 
+define i8 @ret_two() {
+; CHECK-LABEL: define {{[^@]+}}@ret_two()
+; CHECK-NEXT:    ret i8 2
+;
+  ret i8 2
+}
+define i8 @ret_undef() {
+; CHECK-LABEL: define {{[^@]+}}@ret_undef()
+; CHECK-NEXT:    ret i8 undef
+;
+  ret i8 undef
+}
+
+; Verify we collapse undef to a value and return something non-undef here.
+define i8 @undef_collapse_1() {
+; CHECK-LABEL: define {{[^@]+}}@undef_collapse_1()
+; CHECK-NEXT:    ret i8 0
+;
+  %c = call i8 @ret_undef()
+  %s = shl i8 %c, 2
+  ret i8 %s
+}
+
+; Verify we collapse undef to a value and return something non-undef here.
+define i8 @undef_collapse_2() {
+; CHECK-LABEL: define {{[^@]+}}@undef_collapse_2()
+; CHECK-NEXT:    ret i8 0
+;
+  %c = call i8 @ret_two()
+  %s = shl i8 undef, %c
+  ret i8 %s
+}
+
+define i8 @undef_collapse_caller() {
+; OLD_PM-LABEL: define {{[^@]+}}@undef_collapse_caller()
+; OLD_PM-NEXT:    ret i8 0
+;
+; NEW_PM-LABEL: define {{[^@]+}}@undef_collapse_caller()
+; NEW_PM-NEXT:    ret i8 0
+;
+; CGSCC_OLD_PM-LABEL: define {{[^@]+}}@undef_collapse_caller()
+; CGSCC_OLD_PM-NEXT:    [[C1:%.*]] = call i8 @undef_collapse_1()
+; CGSCC_OLD_PM-NEXT:    [[C2:%.*]] = call i8 @undef_collapse_2()
+; CGSCC_OLD_PM-NEXT:    [[A:%.*]] = add i8 [[C1]], [[C2]]
+; CGSCC_OLD_PM-NEXT:    ret i8 [[A]]
+;
+; CGSCC_NEW_PM-LABEL: define {{[^@]+}}@undef_collapse_caller()
+; CGSCC_NEW_PM-NEXT:    [[C1:%.*]] = call i8 @undef_collapse_1()
+; CGSCC_NEW_PM-NEXT:    [[C2:%.*]] = call i8 @undef_collapse_2()
+; CGSCC_NEW_PM-NEXT:    [[A:%.*]] = add i8 [[C1]], [[C2]]
+; CGSCC_NEW_PM-NEXT:    ret i8 [[A]]
+;
+  %c1 = call i8 @undef_collapse_1()
+  %c2 = call i8 @undef_collapse_2()
+  %a = add i8 %c1, %c2
+  ret i8 %a
+}
+
+
 !0 = !{i32 0, i32 10}
 !1 = !{i32 10, i32 100}
 ; CHECK: !0 = !{i32 0, i32 10}
diff --git a/llvm/test/Transforms/Attributor/undefined_behavior.ll b/llvm/test/Transforms/Attributor/undefined_behavior.ll
index bfb7b774002f..cf721611973f 100644
--- a/llvm/test/Transforms/Attributor/undefined_behavior.ll
+++ b/llvm/test/Transforms/Attributor/undefined_behavior.ll
@@ -264,10 +264,9 @@ e:
 ; undef of a previous instruction.
 define i32 @cond_br_on_undef3() {
 ; ATTRIBUTOR-LABEL: @cond_br_on_undef3(
-; ATTRIBUTOR-NEXT:    %cond = icmp ne i32 1, undef
-; ATTRIBUTOR-NEXT:    br i1 %cond, label %t, label %e
+; ATTRIBUTOR-NEXT:    br label %t
 ; ATTRIBUTOR:       t:
-; ATTRIBUTOR-NEXT:    unreachable
+; ATTRIBUTOR-NEXT:    ret i32 1
 ; ATTRIBUTOR:       e:
 ; ATTRIBUTOR-NEXT:    unreachable