From b03dd4be705bcd4e02384f2ea37b893ce477e512 Mon Sep 17 00:00:00 2001
From: Hal Finkel <hfinkel@anl.gov>
Date: Mon, 14 Aug 2017 17:11:43 +0000
Subject: [PATCH] [ValueTracking] Don't delete assumes of side-effectful
 instructions

ValueTracking has to strike a balance when attempting to propagate information
backwards from assumes, because if the information is trivially propagated
backwards, it can appear to LLVM that the assumption is known to be true, and
therefore can be removed.

This is sound (because an assumption has no semantic effect except for causing
UB), but prevents the assume from allowing further optimizations.

The isEphemeralValueOf check exists to try and prevent this issue by not
removing the source of an assumption. This tries to make it a little bit more
general to handle the case of side-effectful instructions, such as in

  %0 = call i1 @get_val()
  %1 = xor i1 %0, true
  call void @llvm.assume(i1 %1)

Patch by Ariel Ben-Yehuda, thanks!

Differential Revision: https://reviews.llvm.org/D36590

llvm-svn: 310859
---
 llvm/lib/Analysis/ValueTracking.cpp        | 14 +++++++-------
 llvm/test/Analysis/ValueTracking/assume.ll | 13 ++++++++++++-
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index b81bf9ec9d1a..d73e5f251192 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -384,13 +384,13 @@ static bool isEphemeralValueOf(const Instruction *I, const Value *E) {
       if (V == E)
         return true;
 
-      EphValues.insert(V);
-      if (const User *U = dyn_cast<User>(V))
-        for (User::const_op_iterator J = U->op_begin(), JE = U->op_end();
-             J != JE; ++J) {
-          if (isSafeToSpeculativelyExecute(*J))
-            WorkSet.push_back(*J);
-        }
+      if (V == I || isSafeToSpeculativelyExecute(V)) {
+       EphValues.insert(V);
+       if (const User *U = dyn_cast<User>(V))
+         for (User::const_op_iterator J = U->op_begin(), JE = U->op_end();
+              J != JE; ++J)
+           WorkSet.push_back(*J);
+      }
     }
   }
 
diff --git a/llvm/test/Analysis/ValueTracking/assume.ll b/llvm/test/Analysis/ValueTracking/assume.ll
index fe0ee53eb416..87ac74c2ad1a 100644
--- a/llvm/test/Analysis/ValueTracking/assume.ll
+++ b/llvm/test/Analysis/ValueTracking/assume.ll
@@ -18,5 +18,16 @@ define i32 @assume_add(i32 %a, i32 %b) {
   ret i32 %t3
 }
 
-declare void @llvm.assume(i1)
 
+define void @assume_not() {
+; CHECK-LABEL: @assume_not(
+entry-block:
+  %0 = call i1 @get_val()
+; CHECK: call void @llvm.assume
+  %1 = xor i1 %0, true
+  call void @llvm.assume(i1 %1)
+  ret void
+}
+
+declare i1 @get_val()
+declare void @llvm.assume(i1)