[llvm] 7ce76b0 - [InstCombine] Fold sdiv exact X, -1<<C --> -(ashr exact X, C)

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2020-08-06T23:37:16+03:00
New Revision: 7ce76b06ec908a85205d4dc7af6e73d5ecc26251

URL: https://github.com/llvm/llvm-project/commit/7ce76b06ec908a85205d4dc7af6e73d5ecc26251
DIFF: https://github.com/llvm/llvm-project/commit/7ce76b06ec908a85205d4dc7af6e73d5ecc26251.diff

LOG: [InstCombine] Fold  sdiv exact X, -1<<C  -->  -(ashr exact X, C)

While that does increases instruction count,
shift is obviously better than a division.

Name: base
Pre: (1<<C1) >= 0
%o0 = shl i8 1, C1
%r = sdiv exact i8 C0, %o0
  =>
%r = ashr exact i8 C0, C1

Name: neg
%o0 = shl i8 -1, C1
%r = sdiv exact i8 C0, %o0
  =>
%t0 = ashr exact i8 C0, C1
%r = sub i8 0, %t0

Name: reverse
Pre: C1 != 0 && C1 u< 8
%t0 = ashr exact i8 C0, C1
%r = sub i8 0, %t0
  =>
%o0 = shl i8 -1, C1
%r = sdiv exact i8 C0, %o0

https://rise4fun.com/Alive/MRplf

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
    llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index cd6f5c7096fc..17c0683af78c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1131,9 +1131,20 @@ Instruction *InstCombinerImpl::visitSDiv(BinaryOperator &I) {
     return new ZExtInst(Builder.CreateICmpEQ(Op0, Op1), I.getType());
 
   // sdiv exact X,  1<<C  -->    ashr exact X, C   iff  1<<C  is non-negative
-  if (I.isExact() && match(Op1, m_Power2()) && match(Op1, m_NonNegative()))
-    return BinaryOperator::CreateExactAShr(
+  // sdiv exact X, -1<<C  -->  -(ashr exact X, C)
+  if (I.isExact() && ((match(Op1, m_Power2()) && match(Op1, m_NonNegative())) ||
+                      match(Op1, m_NegatedPower2()))) {
+    bool DivisorWasNegative = match(Op1, m_NegatedPower2());
+    if (DivisorWasNegative)
+      Op1 = ConstantExpr::getNeg(cast<Constant>(Op1));
+    auto *AShr = BinaryOperator::CreateExactAShr(
         Op0, getLogBase2(I.getType(), cast<Constant>(Op1)), I.getName());
+    if (!DivisorWasNegative)
+      return AShr;
+    Builder.Insert(AShr);
+    AShr->setName(I.getName() + ".neg");
+    return BinaryOperator::CreateNeg(AShr, I.getName());
+  }
 
   const APInt *Op1C;
   if (match(Op1, m_APInt(Op1C))) {

diff  --git a/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll b/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
index 39aa15cb3b54..78858421024e 100644
--- a/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
+++ b/llvm/test/Transforms/InstCombine/sdiv-exact-by-negative-power-of-two.ll
@@ -8,7 +8,8 @@
 
 define i8 @t0(i8 %x) {
 ; CHECK-LABEL: @t0(
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv exact i8 [[X:%.*]], -32
+; CHECK-NEXT:    [[DIV_NEG:%.*]] = ashr exact i8 [[X:%.*]], 5
+; CHECK-NEXT:    [[DIV:%.*]] = sub nsw i8 0, [[DIV_NEG]]
 ; CHECK-NEXT:    ret i8 [[DIV]]
 ;
   %div = sdiv exact i8 %x, -32
@@ -25,7 +26,8 @@ define i8 @n1(i8 %x) {
 
 define <2 x i8> @t2_vec_splat(<2 x i8> %x) {
 ; CHECK-LABEL: @t2_vec_splat(
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv exact <2 x i8> [[X:%.*]], <i8 -32, i8 -32>
+; CHECK-NEXT:    [[DIV_NEG:%.*]] = ashr exact <2 x i8> [[X:%.*]], <i8 5, i8 5>
+; CHECK-NEXT:    [[DIV:%.*]] = sub nsw <2 x i8> zeroinitializer, [[DIV_NEG]]
 ; CHECK-NEXT:    ret <2 x i8> [[DIV]]
 ;
   %div = sdiv exact <2 x i8> %x, <i8 -32, i8 -32>
@@ -34,7 +36,8 @@ define <2 x i8> @t2_vec_splat(<2 x i8> %x) {
 
 define <2 x i8> @t3_vec(<2 x i8> %x) {
 ; CHECK-LABEL: @t3_vec(
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv exact <2 x i8> [[X:%.*]], <i8 -32, i8 -16>
+; CHECK-NEXT:    [[DIV_NEG:%.*]] = ashr exact <2 x i8> [[X:%.*]], <i8 5, i8 4>
+; CHECK-NEXT:    [[DIV:%.*]] = sub <2 x i8> zeroinitializer, [[DIV_NEG]]
 ; CHECK-NEXT:    ret <2 x i8> [[DIV]]
 ;
   %div = sdiv exact <2 x i8> %x, <i8 -32, i8 -16>