[PATCH] D26921: [AArch64LoadStoreOptimizer] Don't treat write to XZR/WZR as a clobber.

[Geoff Berry via llvm-commits]

gberry created this revision.
gberry added reviewers: mcrosier, junbuml, jmolloy, t.p.northover.
gberry added a subscriber: llvm-commits.
Herald added subscribers: rengolin, aemerson.

When searching for load/store instructions to pair/merge don't treat
writes to WZR/XZR as clobbers since they don't change the value read
from WZR/XZR (which is always 0).


https://reviews.llvm.org/D26921

Files:
  lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
  test/CodeGen/AArch64/ldst-opt.ll


Index: test/CodeGen/AArch64/ldst-opt.ll
===================================================================
--- test/CodeGen/AArch64/ldst-opt.ll
+++ test/CodeGen/AArch64/ldst-opt.ll
@@ -1555,3 +1555,16 @@
   store <4 x double> zeroinitializer, <4 x double>* %p
   ret void
 }
+
+; Check that write of xzr doesn't inhibit pariing of xzr stores since
+; it isn't actually clobbered.
+define i1 @no-clobber-zr(i64* %p, i64 %x) nounwind {
+; CHECK-LABEL: no-clobber-zr
+; CHECK: stp xzr, xzr, [x{{[0-9]+}}]
+entry:
+  store i64 0, i64* %p
+  %cond = icmp eq i64 %x, 0
+  %p1 = getelementptr i64, i64* %p, i64 1
+  store i64 0, i64* %p1
+  ret i1 %cond
+}
Index: lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
===================================================================
--- lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
+++ lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
@@ -863,8 +863,10 @@
     if (!Reg)
       continue;
     if (MO.isDef()) {
-      for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
-        ModifiedRegs.set(*AI);
+      // WZR/XZR are not modified even when used as a destination register.
+      if (Reg != AArch64::WZR && Reg != AArch64::XZR)
+        for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
+          ModifiedRegs.set(*AI);
     } else {
       assert(MO.isUse() && "Reg operand not a def and not a use?!?");
       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)


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