[PATCH] D116616: [InstSimplify] use knownbits to fold more udiv/urem

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: nikic, lebedev.ri.
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We could use knownbits on both operands for even more folds (and there are already tests in place for that), but this is enough to recover the example from:
https://github.com/llvm/llvm-project/issues/51934
(the tests are derived from the code in that example)

I am assuming no noticeable compile-time impact from this because udiv/urem are rare opcodes, but I could check that if there is concern.


https://reviews.llvm.org/D116616

Files:
  llvm/lib/Analysis/InstructionSimplify.cpp
  llvm/test/Transforms/InstSimplify/div.ll
  llvm/test/Transforms/InstSimplify/rem.ll


Index: llvm/test/Transforms/InstSimplify/rem.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/rem.ll
+++ llvm/test/Transforms/InstSimplify/rem.ll
@@ -187,8 +187,7 @@
 ; CHECK-LABEL: @urem_dividend_known_smaller_than_constant_divisor2(
 ; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8
 ; CHECK-NEXT:    [[XOR:%.*]] = xor i8 [[T0]], 12
-; CHECK-NEXT:    [[R:%.*]] = urem i8 [[XOR]], 14
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 [[XOR]]
 ;
   %t0 = zext i1 %b to i8
   %xor = xor i8 %t0, 12
@@ -196,6 +195,8 @@
   ret i8 %r
 }
 
+; negative test - dividend can equal 13
+
 define i8 @not_urem_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @not_urem_dividend_known_smaller_than_constant_divisor2(
 ; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8
Index: llvm/test/Transforms/InstSimplify/div.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/div.ll
+++ llvm/test/Transforms/InstSimplify/div.ll
@@ -155,10 +155,7 @@
 
 define i8 @udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @udiv_dividend_known_smaller_than_constant_divisor2(
-; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8
-; CHECK-NEXT:    [[XOR:%.*]] = xor i8 [[T0]], 12
-; CHECK-NEXT:    [[R:%.*]] = udiv i8 [[XOR]], 14
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 0
 ;
   %t0 = zext i1 %b to i8
   %xor = xor i8 %t0, 12
@@ -166,6 +163,8 @@
   ret i8 %r
 }
 
+; negative test - dividend can equal 13
+
 define i8 @not_udiv_dividend_known_smaller_than_constant_divisor2(i1 %b) {
 ; CHECK-LABEL: @not_udiv_dividend_known_smaller_than_constant_divisor2(
 ; CHECK-NEXT:    [[T0:%.*]] = zext i1 [[B:%.*]] to i8
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1078,6 +1078,18 @@
   }
 
   // IsSigned == false.
+
+  // Is the unsigned dividend known to be less than a constant divisor?
+  const APInt *C;
+  if (match(Y, m_APInt(C)) && !C->isZero()) {
+    KnownBits KnownX =
+        computeKnownBits(X, Q.DL, MaxRecurse, Q.AC, Q.CxtI, Q.DT);
+    Optional<bool> IsULT = KnownBits::ult(KnownX, KnownBits::makeConstant(*C));
+    if (IsULT.hasValue() && IsULT.getValue())
+      return true;
+  }
+
+  // Try again for any divisor:
   // Is the dividend unsigned less than the divisor?
   return isICmpTrue(ICmpInst::ICMP_ULT, X, Y, Q, MaxRecurse);
 }


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