[SystemZ] Use "STC;MVC" for memset

Use "STC;MVC" for memsets that are too big for two STCs or MV...Is yet
small enough for a single MVC.  As with memcpy, I'm leaving longer cases
till later.

The number of tests might seem excessive, but f33 & f34 from memset-04.ll
failed the first cut because I'd not added the "?:" on the calculation
of Size1.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185918 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/Target/SystemZ/SystemZISelLowering.cpp b/lib/Target/SystemZ/SystemZISelLowering.cpp
index bf359468e6..b1abc2c3c1 100644
--- a/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -245,6 +245,14 @@ SystemZTargetLowering::SystemZTargetLowering(SystemZTargetMachine &tm)
   // We want to use MVC in preference to even a single load/store pair.
   MaxStoresPerMemcpy = 0;
   MaxStoresPerMemcpyOptSize = 0;
+
+  // The main memset sequence is a byte store followed by an MVC.
+  // Two STC or MV..I stores win over that, but the kind of fused stores
+  // generated by target-independent code don't when the byte value is
+  // variable.  E.g.  "STC <reg>;MHI <reg>,257;STH <reg>" is not better
+  // than "STC;MVC".  Handle the choice in target-specific code instead.
+  MaxStoresPerMemset = 0;
+  MaxStoresPerMemsetOptSize = 0;
 }
 
 bool SystemZTargetLowering::isFPImmLegal(const APFloat &Imm, EVT VT) const {