[X86] Mark all byval parameters as aliased

jmorse · jmorse · commit 4f799c027e09 · 2018-05-08T09:18:01.000Z
This is a fix for PR30290: by marking all byval stack slots as being aliased, the instruction scheduler is more conservative about rescheduling memory accesses to such stack slots as an LLVM Value* might alias it. This fixes errors such as in the patched test case, where reads and writes to a data structure are illegally mixed. This could be fixed better in the future with better analysis for the instruction scheduler to know what Values alias what stack slots. Differential Revision: https://reviews.llvm.org/D45022 llvm-svn: 331749
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2839,7 +2839,11 @@ X86TargetLowering::LowerMemArgument(SDValue Chain, CallingConv::ID CallConv,
   if (Flags.isByVal()) {
     unsigned Bytes = Flags.getByValSize();
     if (Bytes == 0) Bytes = 1; // Don't create zero-sized stack objects.
-    int FI = MFI.CreateFixedObject(Bytes, VA.getLocMemOffset(), isImmutable);
+
+    // FIXME: For now, all byval parameter objects are marked as aliasing. This
+    // can be improved with deeper analysis.
+    int FI = MFI.CreateFixedObject(Bytes, VA.getLocMemOffset(), isImmutable,
+                                   /*isAliased=*/true);
     // Adjust SP offset of interrupt parameter.
     if (CallConv == CallingConv::X86_INTR) {
       MFI.setObjectOffset(FI, Offset);
diff --git a/llvm/test/CodeGen/X86/pr30290.ll b/llvm/test/CodeGen/X86/pr30290.ll
@@ -5,10 +5,6 @@
 ; When broken, five "1" constants are written into the byval %struct.face,
 ; but the subsequent byval read of that struct (call to bar) gets re-ordered
 ; before those writes, illegally.
-;
-; FIXME: the output shown below is the broken output of llc, "movl $1" is
-; scheduled after the copy between byval arguments starts. This will be fixed
-; with the patch in review D45022.
 source_filename = "test.c"
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-linux-gnu"
@@ -26,8 +22,8 @@ define void @foo(%struct.face* byval nocapture align 8) local_unnamed_addr {
 ; CHECK-NEXT:    .cfi_def_cfa_offset 48
 ; CHECK-NEXT:    vmovaps {{.*#+}} xmm0 = [1,1,1,1]
 ; CHECK-NEXT:    vmovaps %xmm0, {{[0-9]+}}(%rsp)
-; CHECK-NEXT:    vmovups {{[0-9]+}}(%rsp), %xmm0
 ; CHECK-NEXT:    movl $1, {{[0-9]+}}(%rsp)
+; CHECK-NEXT:    vmovups {{[0-9]+}}(%rsp), %xmm0
 ; CHECK-NEXT:    vmovups %xmm0, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    vmovaps {{[0-9]+}}(%rsp), %xmm0
 ; CHECK-NEXT:    vmovups %xmm0, (%rsp)
