Teach valuetracking that byval arguments with a specified alignment are

aligned.

Teach memcpyopt to not give up all hope when confonted with an underaligned
memcpy feeding an overaligned byval.  If the *source* of the memcpy can be
determined to be adequeately aligned, or if it can be forced to be, we can
eliminate the memcpy.

This addresses PR9794.  We now compile the example into:

define i32 @f(%struct.p* nocapture byval align 8 %q) nounwind ssp {
entry:
  %call = call i32 @g(%struct.p* byval align 8 %q) nounwind
  ret i32 %call
}

in both x86-64 and x86-32 mode.  We still don't get a tailcall though,
because tailcalls apparently can't handle byval.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131884 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 7b7e39f6c4100243ae2c21e3fa073bc2c1fa3a5e..4f3dc7a7dce93618a87448ce942de0119d1b95d9 100644 (file)
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -131,8 +131,18 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
     }
     return;
   }
+  
+  if (Argument *A = dyn_cast<Argument>(V)) {
+    // Get alignment information off byval arguments if specified in the IR.
+    if (A->hasByValAttr())
+      if (unsigned Align = A->getParamAlignment())
+        KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
+                                                CountTrailingZeros_32(Align));
+    return;
+  }
 
-  KnownZero.clearAllBits(); KnownOne.clearAllBits();   // Start out not knowing anything.
+  // Start out not knowing anything.
+  KnownZero.clearAllBits(); KnownOne.clearAllBits();
 
   if (Depth == MaxDepth || Mask == 0)
     return;  // Limit search depth.

diff --git a/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 04edfd43a285aa88307749b38953430574c71db0..360639ec95bcfe769e62065342cdc200451550e2 100644 (file)
--- a/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -23,6 +23,7 @@
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
 #include "llvm/Analysis/ValueTracking.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/GetElementPtrTypeIterator.h"
 #include "llvm/Support/IRBuilder.h"
@@ -866,12 +867,16 @@ bool MemCpyOpt::processByValArgument(CallSite CS, unsigned ArgNo) {
   if (C1 == 0 || C1->getValue().getZExtValue() < ByValSize)
     return false;
 
-  // Get the alignment of the byval.  If it is greater than the memcpy, then we
-  // can't do the substitution.  If the call doesn't specify the alignment, then
-  // it is some target specific value that we can't know.
+  // Get the alignment of the byval.  If the call doesn't specify the alignment,
+  // then it is some target specific value that we can't know.
   unsigned ByValAlign = CS.getParamAlignment(ArgNo+1);
-  if (ByValAlign == 0 || MDep->getAlignment() < ByValAlign)
-    return false;  
+  if (ByValAlign == 0) return false;
+  
+  // If it is greater than the memcpy, then we check to see if we can force the
+  // source of the memcpy to the alignment we need.  If we fail, we bail out.
+  if (MDep->getAlignment() < ByValAlign &&
+      getOrEnforceKnownAlignment(MDep->getSource(),ByValAlign, TD) < ByValAlign)
+    return false;
   
   // Verify that the copied-from memory doesn't change in between the memcpy and
   // the byval call.

diff --git a/test/Transforms/MemCpyOpt/memcpy.ll b/test/Transforms/MemCpyOpt/memcpy.ll
index b387d32a7d5e8319082715feb698744bb1ba601e..5c6a94ce5c9ff1069305b0b015a13c718199d517 100644 (file)
--- a/test/Transforms/MemCpyOpt/memcpy.ll
+++ b/test/Transforms/MemCpyOpt/memcpy.ll
@@ -109,3 +109,23 @@ define void @test6(i8 *%P) {
 ; CHECK-NEXT: ret void
 }
 
+
+; PR9794 - Should forward memcpy into byval argument even though the memcpy
+; isn't itself 8 byte aligned.
+%struct.p = type { i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32 }
+
+define i32 @test7(%struct.p* nocapture byval align 8 %q) nounwind ssp {
+entry:
+  %agg.tmp = alloca %struct.p, align 4
+  %tmp = bitcast %struct.p* %agg.tmp to i8*
+  %tmp1 = bitcast %struct.p* %q to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %tmp, i8* %tmp1, i64 48, i32 4, i1 false)
+  %call = call i32 @g(%struct.p* byval align 8 %agg.tmp) nounwind
+  ret i32 %call
+; CHECK: @test7
+; CHECK: call i32 @g(%struct.p* byval align 8 %q) nounwind
+}
+
+declare i32 @g(%struct.p* byval align 8)
+
+