[llvm] r221318 - Analysis: Make isSafeToSpeculativelyExecute fire less for divides

[David Majnemer]

Author: majnemer
Date: Tue Nov  4 17:49:08 2014
New Revision: 221318

URL: http://llvm.org/viewvc/llvm-project?rev=221318&view=rev
Log:
Analysis: Make isSafeToSpeculativelyExecute fire less for divides

Divides and remainder operations do not behave like other operations
when they are given poison: they turn into undefined behavior.

It's really hard to know if the operands going into a div are or are not
poison.  Because of this, we should only choose to speculate if there
are constant operands which we can easily reason about.

This fixes PR21412.

Modified:
    llvm/trunk/lib/Analysis/ValueTracking.cpp
    llvm/trunk/test/Transforms/LICM/speculate.ll

Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=221318&r1=221317&r2=221318&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Tue Nov  4 17:49:08 2014
@@ -2549,23 +2549,31 @@ bool llvm::isSafeToSpeculativelyExecute(
   default:
     return true;
   case Instruction::UDiv:
-  case Instruction::URem:
-    // x / y is undefined if y == 0, but calculations like x / 3 are safe.
-    return isKnownNonZero(Inst->getOperand(1), TD);
+  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;
+  }
   case Instruction::SDiv:
   case Instruction::SRem: {
-    Value *Op = Inst->getOperand(1);
-    // x / y is undefined if y == 0
-    if (!isKnownNonZero(Op, TD))
-      return false;
-    // x / y might be undefined if y == -1
-    unsigned BitWidth = getBitWidth(Op->getType(), TD);
-    if (BitWidth == 0)
-      return false;
-    APInt KnownZero(BitWidth, 0);
-    APInt KnownOne(BitWidth, 0);
-    computeKnownBits(Op, KnownZero, KnownOne, TD);
-    return !!KnownZero;
+    // x / y is undefined if y == 0 or x == INT_MIN and y == -1
+    const APInt *X, *Y;
+    if (match(Inst->getOperand(1), m_APInt(Y))) {
+      if (*Y != 0) {
+        if (*Y == -1) {
+          // The numerator can't be MinSignedValue if the denominator is -1.
+          if (match(Inst->getOperand(0), m_APInt(X)))
+            return !Y->isMinSignedValue();
+          // The numerator *might* be MinSignedValue.
+          return false;
+        }
+        // The denominator is not 0 or -1, it's safe to proceed.
+        return true;
+      }
+    }
+    return false;
   }
   case Instruction::Load: {
     const LoadInst *LI = cast<LoadInst>(Inst);

Modified: llvm/trunk/test/Transforms/LICM/speculate.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LICM/speculate.ll?rev=221318&r1=221317&r2=221318&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LICM/speculate.ll (original)
+++ llvm/trunk/test/Transforms/LICM/speculate.ll Tue Nov  4 17:49:08 2014
@@ -3,12 +3,11 @@
 ; UDiv is safe to speculate if the denominator is known non-zero.
 
 ; CHECK-LABEL: @safe_udiv(
-; CHECK:      %div = udiv i64 %x, %or
+; CHECK:      %div = udiv i64 %x, 2
 ; CHECK-NEXT: br label %for.body
 
 define void @safe_udiv(i64 %x, i64 %m, i64 %n, i32* %p, i64* %q) nounwind {
 entry:
-  %or = or i64 %m, 1
   br label %for.body
 
 for.body:                                         ; preds = %entry, %for.inc
@@ -19,7 +18,7 @@ for.body:
   br i1 %tobool, label %for.inc, label %if.then
 
 if.then:                                          ; preds = %for.body
-  %div = udiv i64 %x, %or
+  %div = udiv i64 %x, 2
   %arrayidx1 = getelementptr inbounds i64* %q, i64 %i.02
   store i64 %div, i64* %arrayidx1, align 8
   br label %for.inc
@@ -69,13 +68,12 @@ for.end:
 ; known to have at least one zero bit.
 
 ; CHECK-LABEL: @safe_sdiv(
-; CHECK:      %div = sdiv i64 %x, %or
+; CHECK:      %div = sdiv i64 %x, 2
 ; CHECK-NEXT: br label %for.body
 
 define void @safe_sdiv(i64 %x, i64 %m, i64 %n, i32* %p, i64* %q) nounwind {
 entry:
   %and = and i64 %m, -3
-  %or = or i64 %and, 1
   br label %for.body
 
 for.body:                                         ; preds = %entry, %for.inc
@@ -86,7 +84,7 @@ for.body:
   br i1 %tobool, label %for.inc, label %if.then
 
 if.then:                                          ; preds = %for.body
-  %div = sdiv i64 %x, %or
+  %div = sdiv i64 %x, 2
   %arrayidx1 = getelementptr inbounds i64* %q, i64 %i.02
   store i64 %div, i64* %arrayidx1, align 8
   br label %for.inc