From 403539ab6483aee5b7b6f83362a9f9690e503f46 Mon Sep 17 00:00:00 2001
From: Bjorn Pettersson <bjorn.a.pettersson@ericsson.com>
Date: Thu, 6 Oct 2016 09:56:21 +0000
Subject: [PATCH] [ValueTracking] Teach computeKnownBits and ComputeNumSignBits
 to look through ExtractElement.

Summary:
The computeKnownBits and ComputeNumSignBits functions in ValueTracking can now do a simple look-through of ExtractElement.

Reviewers: majnemer, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24955

llvm-svn: 283434
---
 lib/Analysis/ValueTracking.cpp                | 14 ++++++++++
 .../ValueTracking/signbits-extract-elt.ll     | 28 +++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 test/Analysis/ValueTracking/signbits-extract-elt.ll

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index e409edb28fd..64a900d2960 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -1389,6 +1389,13 @@ static void computeKnownBitsFromOperator(const Operator *I, APInt &KnownZero,
       }
     }
     break;
+  case Instruction::ExtractElement:
+    // Look through extract element. At the moment we keep this simple and skip
+    // tracking the specific element. But at least we might find information
+    // valid for all elements of the vector (for example if vector is sign
+    // extended, shifted, etc).
+    computeKnownBits(I->getOperand(0), KnownZero, KnownOne, Depth + 1, Q);
+    break;
   case Instruction::ExtractValue:
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I->getOperand(0))) {
       const ExtractValueInst *EVI = cast<ExtractValueInst>(I);
@@ -2220,6 +2227,13 @@ unsigned ComputeNumSignBits(const Value *V, unsigned Depth, const Query &Q) {
     // FIXME: it's tricky to do anything useful for this, but it is an important
     // case for targets like X86.
     break;
+
+  case Instruction::ExtractElement:
+    // Look through extract element. At the moment we keep this simple and skip
+    // tracking the specific element. But at least we might find information
+    // valid for all elements of the vector (for example if vector is sign
+    // extended, shifted, etc).
+    return ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
   }
 
   // Finally, if we can prove that the top bits of the result are 0's or 1's,
diff --git a/test/Analysis/ValueTracking/signbits-extract-elt.ll b/test/Analysis/ValueTracking/signbits-extract-elt.ll
new file mode 100644
index 00000000000..e46c0125699
--- /dev/null
+++ b/test/Analysis/ValueTracking/signbits-extract-elt.ll
@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instsimplify -S | FileCheck %s
+
+; If computeKnownBits can do a simple look-through for extractelement
+; then instsimplify will know that %elt1 is non-negative at icmp.
+define i1 @computeKnownBits_look_through_extractelt(<2 x i8> %vecin) {
+; CHECK-LABEL: @computeKnownBits_look_through_extractelt(
+; CHECK-NEXT:    ret i1 false
+;
+  %vec = zext <2 x i8> %vecin to <2 x i32>
+  %elt1 = extractelement <2 x i32> %vec, i32 1
+  %bool = icmp slt i32 %elt1, 0
+  ret i1 %bool
+}
+
+; If computeNumSignBits can do a simple look-through for extractelement
+; then instsimplify will remove the ashr.
+define i32 @computeNumSignBits_look_through_extractelt(<2 x i1> %vecin) {
+; CHECK-LABEL: @computeNumSignBits_look_through_extractelt(
+; CHECK-NEXT:    [[VEC:%.*]] = sext <2 x i1> [[VEC:%.*]]in to <2 x i32>
+; CHECK-NEXT:    [[ELT0:%.*]] = extractelement <2 x i32> [[VEC]], i32 0
+; CHECK-NEXT:    ret i32 [[ELT0]]
+;
+  %vec = sext <2 x i1> %vecin to <2 x i32>
+  %elt0 = extractelement <2 x i32> %vec, i32 0
+  %ashr = ashr i32 %elt0, 5
+  ret i32 %ashr
+}