[PATCH] D26818: Bug fix and a proposed improvement in integer division expansion

[Dmitri Shtilman via llvm-commits]

dshtilman set the repository for this revision to rL LLVM.
dshtilman updated this revision to Diff 78432.

Repository:
  rL LLVM

https://reviews.llvm.org/D26818

Files:
  lib/Transforms/Utils/IntegerDivision.cpp


Index: lib/Transforms/Utils/IntegerDivision.cpp
===================================================================
--- lib/Transforms/Utils/IntegerDivision.cpp
+++ lib/Transforms/Utils/IntegerDivision.cpp
@@ -182,6 +182,8 @@
   Function *F = IBB->getParent();
   Function *CTLZ = Intrinsic::getDeclaration(F->getParent(), Intrinsic::ctlz,
                                              DivTy);
+  Function *CTTZ = Intrinsic::getDeclaration(F->getParent(), Intrinsic::cttz,
+                                             DivTy);
 
   // Our CFG is going to look like:
   // +---------------------+
@@ -234,7 +236,7 @@
   // Same instructions are generated for both i32 (msb 31) and i64 (msb 63).
 
   // First off, check for special cases: dividend or divisor is zero, divisor
-  // is greater than dividend, and divisor is 1.
+  // is greater than dividend, and divisor is a power of 2.
   // ; special-cases:
   // ;   %ret0_1      = icmp eq i32 %divisor, 0
   // ;   %ret0_2      = icmp eq i32 %dividend, 0
@@ -244,8 +246,11 @@
   // ;   %sr          = sub nsw i32 %tmp0, %tmp1
   // ;   %ret0_4      = icmp ugt i32 %sr, 31
   // ;   %ret0        = or i1 %ret0_3, %ret0_4
-  // ;   %retDividend = icmp eq i32 %sr, 31
-  // ;   %retVal      = select i1 %ret0, i32 0, i32 %dividend
+  // ;   %tmp14       = tail call i32 @llvm.cttz.i32(i32 %divisor, i1 true)
+  // ;   %tmp15       = add i32 %tmp0, %tmp14
+  // ;   %retDividend = icmp eq i32 %tmp15, 31
+  // ;   %dividend_1  = lshr i32 %dividend, %tmp14
+  // ;   %retVal      = select i1 %ret0, i32 0, i32 %dividend_1
   // ;   %earlyRet    = or i1 %ret0, %retDividend
   // ;   br i1 %earlyRet, label %end, label %bb1
   Builder.SetInsertPoint(SpecialCases);
@@ -257,8 +262,11 @@
   Value *SR          = Builder.CreateSub(Tmp0, Tmp1);
   Value *Ret0_4      = Builder.CreateICmpUGT(SR, MSB);
   Value *Ret0        = Builder.CreateOr(Ret0_3, Ret0_4);
-  Value *RetDividend = Builder.CreateICmpEQ(SR, MSB);
-  Value *RetVal      = Builder.CreateSelect(Ret0, Zero, Dividend);
+  Value *Tmp14       = Builder.CreateCall(CTTZ, {Divisor, True});
+  Value *Tmp15       = Builder.CreateAdd(Tmp0, Tmp14);
+  Value *RetDividend = Builder.CreateICmpEQ(Tmp15, MSB);
+  Value *Dividend_1  = Builder.CreateLShr(Dividend, Tmp14);
+  Value *RetVal      = Builder.CreateSelect(Ret0, Zero, Dividend_1);
   Value *EarlyRet    = Builder.CreateOr(Ret0, RetDividend);
   Builder.CreateCondBr(EarlyRet, End, BB1);
 


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