[PATCH] D10904: Fix Integer Division Expansion special case of divide by 1

[Aditya Nandakumar]

aditya_nandakumar created this revision.
aditya_nandakumar added a reviewer: llvm-commits-list.
aditya_nandakumar set the repository for this revision to rL LLVM.

I think the handling of special case of divide by 1 is incorrect in udivsi3.c (compiler-rt) and it’s equivalent in llvm/lib/Transforms/Utils/IntegerDivision.cpp

I think we want to check if the clz(divisor) == (Num_word_bits - 1) rather than checking
(clz(divisor) - clz(dividend)) == (Num_word_bits - 1)

I also think compiler-rt should be updated - 
http://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/builtins/udivsi3.c

sr = __builtin_clz(d) - __builtin_clz(n);
    /* 0 <= sr <= n_uword_bits - 1 or sr large */
    if (sr > n_uword_bits - 1)  /* d > r */
        return 0;
    if (sr == n_uword_bits - 1)  /* d == 1 */
        return n;
The last line should be replaced with 
	
	if ( __buildin_clz(d) == n_uword_bits - 1)
		return n;

rL LLVM

http://reviews.llvm.org/D10904

Files:
  lib/Transforms/Utils/IntegerDivision.cpp

Index: lib/Transforms/Utils/IntegerDivision.cpp
===================================================================
--- lib/Transforms/Utils/IntegerDivision.cpp
+++ lib/Transforms/Utils/IntegerDivision.cpp
@@ -244,7 +244,7 @@
   // ;   %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
+  // ;   %retDividend = icmp eq i32 %tmp0, 31
   // ;   %retVal      = select i1 %ret0, i32 0, i32 %dividend
   // ;   %earlyRet    = or i1 %ret0, %retDividend
   // ;   br i1 %earlyRet, label %end, label %bb1
@@ -257,7 +257,7 @@
   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 *RetDividend = Builder.CreateICmpEQ(Tmp0, MSB);
   Value *RetVal      = Builder.CreateSelect(Ret0, Zero, Dividend);
   Value *EarlyRet    = Builder.CreateOr(Ret0, RetDividend);
   Builder.CreateCondBr(EarlyRet, End, BB1);


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