Fix PR 17368: disable vector mul distribution for square of add/sub for ARM

Generally, it is desirable to distribute (a + b) * c to a*c + b*c for
ARM with VMLx forwarding, where a, b and c are vectors.
However, for (a + b)*(a + b), distribution will result in one extra
instruction.
With distribution:
  x = a + b (add)
  y = a * x (mul)
  z = y + b * y (mla)

Without distribution:
  x = a + b (add)
  z = x * x (mul)

This patch checks if a mul is a square of add/sub. If yes, skip
distribution.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191410 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 0 deletions

diff --git a/test/CodeGen/ARM/vmul.ll b/test/CodeGen/ARM/vmul.ll
index 5e5e99bc2f9..de329acdf3c 100644
--- a/test/CodeGen/ARM/vmul.ll
+++ b/test/CodeGen/ARM/vmul.ll
@@ -515,6 +515,17 @@ entry:
   ret void
 }
 
+define <8 x i8> @no_distribute(<8 x i8> %a, <8 x i8> %b) nounwind {
+entry:
+; CHECK: no_distribute
+; CHECK: vadd.i8
+; CHECK: vmul.i8
+; CHECK-NOT: vmla.i8
+  %0 = add <8 x i8> %a, %b
+  %1 = mul <8x i8> %0, %0
+  ret <8 x i8> %1
+}
+
 ; If one operand has a zero-extend and the other a sign-extend, vmull
 ; cannot be used.
 define i16 @vmullWithInconsistentExtensions(<8 x i8> %vec) {