[PATCH] D35035: [InstCombine] Prevent memcpy generation for small data size

[DIVYA SHANMUGHAN via Phabricator via llvm-commits]

DIVYA updated this revision to Diff 110467.

https://reviews.llvm.org/D35035

Files:
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  test/DebugInfo/X86/array2.ll


Index: test/DebugInfo/X86/array2.ll
===================================================================
--- test/DebugInfo/X86/array2.ll
+++ test/DebugInfo/X86/array2.ll
@@ -53,7 +53,7 @@
   call void @llvm.dbg.declare(metadata i8*** %argv.addr, metadata !25, metadata !DIExpression()), !dbg !24
   call void @llvm.dbg.declare(metadata [4 x i32]* %array, metadata !26, metadata !DIExpression()), !dbg !30
   %0 = bitcast [4 x i32]* %array to i8*, !dbg !30
-  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast ([4 x i32]* @main.array to i8*), i64 16, i32 16, i1 false), !dbg !30
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast ([4 x i32]* @main.array to i8*), i64 128, i32 16, i1 false), !dbg !30
   %arraydecay = getelementptr inbounds [4 x i32], [4 x i32]* %array, i32 0, i32 0, !dbg !31
   call void @f(i32* %arraydecay), !dbg !31
   %arrayidx = getelementptr inbounds [4 x i32], [4 x i32]* %array, i32 0, i64 0, !dbg !32
Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -182,8 +182,6 @@
     return MI;
   }
 
-  // If MemCpyInst length is 1/2/4/8 bytes then replace memcpy with
-  // load/store.
   ConstantInt *MemOpLength = dyn_cast<ConstantInt>(MI->getArgOperand(2));
   if (!MemOpLength) return nullptr;
 
@@ -194,8 +192,16 @@
   uint64_t Size = MemOpLength->getLimitedValue();
   assert(Size && "0-sized memory transferring should be removed already.");
 
-  if (Size > 8 || (Size&(Size-1)))
-    return nullptr;  // If not 1/2/4/8 bytes, exit.
+
+  // Since we don't have perfect knowledge here, make some assumptions: assume
+  // the maximum allowed stores for memcpy operation is the same size as the 
+  // largest legal integer size. 
+  unsigned LargestInt = DL.getLargestLegalIntTypeSizeInBits();
+  if (LargestInt == 0)
+    LargestInt = 32;
+
+  if (Size > 2*LargestInt/8 || (Size&(Size-1)))
+    return nullptr;  
 
   // Use an integer load+store unless we can find something better.
   unsigned SrcAddrSp =


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