LoopVectorizer: integer division is not a reduction operation

Don't classify idiv/udiv as a reduction operation. Integer division is lossy.
For example : (1 / 2) * 4 != 4/2.

Example:

int a[] = { 2, 5, 2, 2}
int x = 80;

for()
  x /= a[i];

Scalar:
  x /= 2 // = 40
  x /= 5 // = 8
  x /= 2 // = 4
  x /= 2 // = 2

Vectorized:

 <80, 1> / <2,5> //= <40,0>
 <40, 0> / <2,2> //= <20,0>

 20*0 = 0

radar://13640654

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179381 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index acf2b819b81377702f52fc4c1035f1074bf4f25f..238fc8bfb701617efcd8b4ef13858d1d781ff161 100644 (file)
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2833,8 +2833,6 @@ LoopVectorizationLegality::isReductionInstr(Instruction *I,
   case Instruction::Sub:
   case Instruction::Add:
     return Kind == RK_IntegerAdd;
-  case Instruction::SDiv:
-  case Instruction::UDiv:
   case Instruction::Mul:
     return Kind == RK_IntegerMult;
   case Instruction::And:

diff --git a/test/Transforms/LoopVectorize/no_idiv_reduction.ll b/test/Transforms/LoopVectorize/no_idiv_reduction.ll
new file mode 100644 (file)
index 0000000..cdfb3fd
--- /dev/null
+++ b/test/Transforms/LoopVectorize/no_idiv_reduction.ll
@@ -0,0 +1,24 @@
+; RUN: opt -loop-vectorize -force-vector-width=2 -force-vector-unroll=1 -S < %s | FileCheck %s
+@a = common global [128 x i32] zeroinitializer, align 16
+
+;; Must not vectorize division reduction. Division is lossy.
+define i32 @g() {
+entry:
+  br label %for.body
+
+for.body:
+  ; CHECK: @g
+  ; CHECK-NOT: sdiv <2 x i32>
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %r.05 = phi i32 [ 80, %entry ], [ %div, %for.body ]
+  %arrayidx = getelementptr inbounds [128 x i32]* @a, i64 0, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %div = sdiv i32 %r.05, %0
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, 1024
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret i32 %div
+}