From fbc7fcf5ae26fad7db3e7c861fc6447e02b39cf0 Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Sun, 30 Apr 2023 10:16:55 -0500
Subject: [PATCH] [ValueTracking] Use knownbits interface for determining if
 `div`/`rem` are safe to speculate

This just replaces the exact constant requirements with known-bits
which can prove better results.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149423
---
 llvm/lib/Analysis/ValueTracking.cpp        | 27 +++++++++++++--------------
 llvm/test/Transforms/LICM/speculate-div.ll |  6 +++---
 2 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index a5259ba..9972a43 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -6035,29 +6035,28 @@ bool llvm::isSafeToSpeculativelyExecuteWithOpcode(
   case Instruction::UDiv:
   case Instruction::URem: {
     // x / y is undefined if y == 0.
-    const APInt *V;
-    if (match(Inst->getOperand(1), m_APInt(V)))
-      return *V != 0;
-    return false;
+    const DataLayout &DL = Inst->getModule()->getDataLayout();
+    return isKnownNonZero(Inst->getOperand(1), DL, /*Depth*/ 0, AC, CtxI, DT);
   }
   case Instruction::SDiv:
   case Instruction::SRem: {
     // x / y is undefined if y == 0 or x == INT_MIN and y == -1
-    const APInt *Numerator, *Denominator;
-    if (!match(Inst->getOperand(1), m_APInt(Denominator)))
-      return false;
+    const DataLayout &DL = Inst->getModule()->getDataLayout();
+    KnownBits KnownDenominator =
+        computeKnownBits(Inst->getOperand(1), DL, /*Depth*/ 0, AC, CtxI, DT);
     // We cannot hoist this division if the denominator is 0.
-    if (*Denominator == 0)
+    if (!KnownDenominator.isNonZero())
       return false;
+
     // It's safe to hoist if the denominator is not 0 or -1.
-    if (!Denominator->isAllOnes())
+    if (!KnownDenominator.Zero.isZero())
       return true;
-    // At this point we know that the denominator is -1.  It is safe to hoist as
+
+    // At this point denominator may be -1.  It is safe to hoist as
     // long we know that the numerator is not INT_MIN.
-    if (match(Inst->getOperand(0), m_APInt(Numerator)))
-      return !Numerator->isMinSignedValue();
-    // The numerator *might* be MinSignedValue.
-    return false;
+    KnownBits KnownNumerator =
+        computeKnownBits(Inst->getOperand(0), DL, /*Depth*/ 0, AC, CtxI, DT);
+    return !KnownNumerator.getSignedMinValue().isMinSignedValue();
   }
   case Instruction::Load: {
     const LoadInst *LI = dyn_cast<LoadInst>(Inst);
diff --git a/llvm/test/Transforms/LICM/speculate-div.ll b/llvm/test/Transforms/LICM/speculate-div.ll
index 50e755e..e0e2c5e 100644
--- a/llvm/test/Transforms/LICM/speculate-div.ll
+++ b/llvm/test/Transforms/LICM/speculate-div.ll
@@ -51,10 +51,10 @@ define void @sdiv_ok(i16 %n, i16 %xx) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[XO:%.*]] = or i16 [[XX:%.*]], 1
 ; CHECK-NEXT:    [[X:%.*]] = and i16 [[XO]], 123
+; CHECK-NEXT:    [[DIV:%.*]] = sdiv i16 [[N:%.*]], [[X]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    call void @maythrow()
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv i16 [[N:%.*]], [[X]]
 ; CHECK-NEXT:    call void @use(i16 [[DIV]])
 ; CHECK-NEXT:    br label [[LOOP]]
 ;
@@ -74,10 +74,10 @@ define void @srem_ok2(i16 %nn, i16 %xx) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[N:%.*]] = and i16 [[NN:%.*]], 123
 ; CHECK-NEXT:    [[X:%.*]] = or i16 [[XX:%.*]], 1
+; CHECK-NEXT:    [[DIV:%.*]] = srem i16 [[N]], [[X]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    call void @maythrow()
-; CHECK-NEXT:    [[DIV:%.*]] = srem i16 [[N]], [[X]]
 ; CHECK-NEXT:    call void @use(i16 [[DIV]])
 ; CHECK-NEXT:    br label [[LOOP]]
 ;
@@ -117,10 +117,10 @@ define void @udiv_ok(i16 %n, i16 %xx) {
 ; CHECK-LABEL: @udiv_ok(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[X:%.*]] = or i16 [[XX:%.*]], 1
+; CHECK-NEXT:    [[DIV:%.*]] = udiv i16 [[N:%.*]], [[X]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    call void @maythrow()
-; CHECK-NEXT:    [[DIV:%.*]] = udiv i16 [[N:%.*]], [[X]]
 ; CHECK-NEXT:    call void @use(i16 [[DIV]])
 ; CHECK-NEXT:    br label [[LOOP]]
 ;
-- 
2.7.4