It is illegal to transform (sdiv (ashr X c1) c2) -> (sdiv x (2^c1 * c2)),

because C always rounds towards zero.

Thanks Dirk and Ben.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162899 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 2119115ca71bf9338c79e2d25f0e387bb4ed40c1..003dc55a03f8b4f12dab4c9fd6aa82eaff8f9ed4 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -543,16 +543,6 @@ Instruction *InstCombiner::visitSDiv(BinaryOperator &I) {
                                           ConstantExpr::getNeg(RHS));
   }
 
-  // Sdiv ((Ashl x, C1) , C2) ->  x / (C2 * 1<<C1);
-  if (ConstantInt *C2 = dyn_cast<ConstantInt>(Op1)) {
-    Value *X;
-    ConstantInt *C1;
-    if (match(Op0, m_AShr(m_Value(X), m_ConstantInt(C1)))) {
-      APInt NC = C2->getValue().shl(C1->getLimitedValue(C1->getBitWidth()-1));
-      return BinaryOperator::CreateSDiv(X, Builder->getInt(NC));
-    }
-  }
-
   // If the sign bits of both operands are zero (i.e. we can prove they are
   // unsigned inputs), turn this into a udiv.
   if (I.getType()->isIntegerTy()) {

diff --git a/test/Transforms/InstCombine/2012-08-28-udiv_ashl.ll b/test/Transforms/InstCombine/2012-08-28-udiv_ashl.ll
index a35a816219a0517e9ebeb4b783d1d8432a472af9..4efaf8c172556478f7169f2b28a11214adbe6d16 100644 (file)
--- a/test/Transforms/InstCombine/2012-08-28-udiv_ashl.ll
+++ b/test/Transforms/InstCombine/2012-08-28-udiv_ashl.ll
@@ -15,15 +15,6 @@ entry:
   ret i32 %div1
 }
 
-; CHECK: @sdiv400
-; CHECK: sdiv i32 %x, 400
-; CHECK: ret
-define i32 @sdiv400(i32 %x) {
-entry:
-  %div = ashr i32 %x, 2
-  %div1 = sdiv i32 %div, 100
-  ret i32 %div1
-}
 
 ; CHECK: @udiv400_no
 ; CHECK: ashr
@@ -59,22 +50,6 @@ define i80 @udiv_i80(i80 %x) {
   ret i80 %div1
 }
 
-; CHECK: @sdiv_i80
-; CHECK: sdiv i80 %x, 400
-; CHECK: ret
-define i80 @sdiv_i80(i80 %x) {
-  %div = ashr i80 %x, 2
-  %div1 = sdiv i80 %div, 100
-  ret i80 %div1
-}
-
-
-define i32 @no_crash_notconst_sdiv(i32 %x, i32 %notconst) {
-  %div = ashr i32 %x, %notconst
-  %div1 = sdiv i32 %div, 100
-  ret i32 %div1
-}
-
 define i32 @no_crash_notconst_udiv(i32 %x, i32 %notconst) {
   %div = lshr i32 %x, %notconst
   %div1 = udiv i32 %div, 100