[llvm] fe15290 - [InstCombine] fold exact sdiv to ashr

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2022-10-08T09:23:46-04:00
New Revision: fe15290e0cf5d2bcdefca2e81ef6ff8155a2f7a8

URL: https://github.com/llvm/llvm-project/commit/fe15290e0cf5d2bcdefca2e81ef6ff8155a2f7a8
DIFF: https://github.com/llvm/llvm-project/commit/fe15290e0cf5d2bcdefca2e81ef6ff8155a2f7a8.diff

LOG: [InstCombine] fold exact sdiv to ashr

sdiv exact X, (1<<ShAmt) --> ashr exact X, ShAmt (if shl is non-negative)

https://alive2.llvm.org/ce/z/kB6VF7

It would probably be better to use ValueTracking to replace this
and the existing transform above it, but the analysis does not
account for the no-wrap properly, and it's not immediately clear
to me how to fix it.

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 69625c082c5c7..d9fcd436b00e0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1199,6 +1199,11 @@ Instruction *InstCombinerImpl::visitSDiv(BinaryOperator &I) {
       return BinaryOperator::CreateExactAShr(Op0, C);
     }
 
+    // sdiv exact X, (1<<ShAmt) --> ashr exact X, ShAmt (if shl is non-negative)
+    Value *ShAmt;
+    if (match(Op1, m_NSWShl(m_One(), m_Value(ShAmt))))
+      return BinaryOperator::CreateExactAShr(Op0, ShAmt);
+
     // sdiv exact X, -1<<C --> -(ashr exact X, C)
     if (match(Op1, m_NegatedPower2())) {
       Constant *NegPow2C = ConstantExpr::getNeg(cast<Constant>(Op1));

diff  --git a/llvm/test/Transforms/InstCombine/sdiv-exact-by-power-of-two.ll b/llvm/test/Transforms/InstCombine/sdiv-exact-by-power-of-two.ll
index 8bc29d04118be..ff650de2c5c80 100644
--- a/llvm/test/Transforms/InstCombine/sdiv-exact-by-power-of-two.ll
+++ b/llvm/test/Transforms/InstCombine/sdiv-exact-by-power-of-two.ll
@@ -71,16 +71,13 @@ define <2 x i8> @n6_vec_negative(<2 x i8> %x) {
 }
 
 define i8 @shl1_nsw(i8 %x, i8 %y) {
-; CHECK-LABEL: @shl1_nsw(
-; CHECK-NEXT:    [[SHL:%.*]] = shl nuw nsw i8 1, [[Y:%.*]]
-; CHECK-NEXT:    [[DIV:%.*]] = sdiv exact i8 [[X:%.*]], [[SHL]]
-; CHECK-NEXT:    ret i8 [[DIV]]
-;
   %shl = shl nsw i8 1, %y
   %div = sdiv exact i8 %x, %shl
   ret i8 %div
 }
 
+; negative test - must have nsw
+
 define i8 @shl1_nuw(i8 %x, i8 %y) {
 ; CHECK-LABEL: @shl1_nuw(
 ; CHECK-NEXT:    [[SHL:%.*]] = shl nuw i8 1, [[Y:%.*]]
@@ -91,3 +88,11 @@ define i8 @shl1_nuw(i8 %x, i8 %y) {
   %div = sdiv exact i8 %x, %shl
   ret i8 %div
 }
+
+; negative test - must have exact
+
+define i8 @shl1_nsw_not_exact(i8 %x, i8 %y) {
+  %shl = shl nsw i8 1, %y
+  %div = sdiv i8 %x, %shl
+  ret i8 %div
+}