[llvm] r210444 - [SeparateConstOffsetFromGEP] inbounds zext => sext for better splitting

Jingyue Wu jingyue at google.com
Sun Jun 8 16:49:35 PDT 2014


Author: jingyue
Date: Sun Jun  8 18:49:34 2014
New Revision: 210444

URL: http://llvm.org/viewvc/llvm-project?rev=210444&view=rev
Log:
[SeparateConstOffsetFromGEP] inbounds zext => sext for better splitting

For each array index that is in the form of zext(a), convert it to sext(a)
if we can prove zext(a) <= max signed value of typeof(a). The conversion
helps to split zext(x + y) into sext(x) + sext(y).

Reviewed in http://reviews.llvm.org/D4060


Modified:
    llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
    llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
    llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll

Modified: llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp?rev=210444&r1=210443&r2=210444&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp Sun Jun  8 18:49:34 2014
@@ -268,8 +268,27 @@ class SeparateConstOffsetFromGEP : publi
   /// This canonicalization is very likely already done in clang and
   /// instcombine. Therefore, the program will probably remain the same.
   ///
+  /// Returns true if the module changes.
+  ///
   /// Verified in @i32_add in split-gep.ll
   bool canonicalizeArrayIndicesToPointerSize(GetElementPtrInst *GEP);
+  /// For each array index that is in the form of zext(a), convert it to sext(a)
+  /// if we can prove zext(a) <= max signed value of typeof(a). We prefer
+  /// sext(a) to zext(a), because in the special case where x + y >= 0 and
+  /// (x >= 0 or y >= 0), function CanTraceInto can split sext(x + y),
+  /// while no such case exists for zext(x + y).
+  ///
+  /// Note that
+  ///   zext(x + y) = zext(x) + zext(y)
+  /// is wrong, e.g.,
+  ///   zext i32(UINT_MAX + 1) to i64 !=
+  ///   (zext i32 UINT_MAX to i64) + (zext i32 1 to i64)
+  ///
+  /// Returns true if the module changes.
+  ///
+  /// Verified in @inbounds_zext_add in split-gep.ll and @sum_of_array3 in
+  /// split-gep-and-gvn.ll
+  bool convertInBoundsZExtToSExt(GetElementPtrInst *GEP);
 
   const DataLayout *DL;
 };
@@ -403,7 +422,6 @@ APInt ConstantOffsetExtractor::find(Valu
     // sext(zext(a)) = zext(a). Verified in @sext_zext in split-gep.ll.
     //
     // Clear the NonNegative flag, because zext(a) >= 0 does not imply a >= 0.
-    // TODO: if zext(a) < 2 ^ (bitwidth(a) - 1), we can prove a >= 0.
     ConstantOffset =
         find(U->getOperand(0), /* SignExtended */ false,
              /* ZeroExtended */ true, /* NonNegative */ false).zext(BitWidth);
@@ -595,6 +613,43 @@ bool SeparateConstOffsetFromGEP::canonic
   return Changed;
 }
 
+bool
+SeparateConstOffsetFromGEP::convertInBoundsZExtToSExt(GetElementPtrInst *GEP) {
+  if (!GEP->isInBounds())
+    return false;
+
+  // TODO: consider alloca
+  GlobalVariable *UnderlyingObject =
+      dyn_cast<GlobalVariable>(GEP->getPointerOperand());
+  if (UnderlyingObject == nullptr)
+    return false;
+
+  uint64_t ObjectSize =
+      DL->getTypeAllocSize(UnderlyingObject->getType()->getElementType());
+  gep_type_iterator GTI = gep_type_begin(*GEP);
+  bool Changed = false;
+  for (User::op_iterator I = GEP->op_begin() + 1, E = GEP->op_end(); I != E;
+       ++I, ++GTI) {
+    if (isa<SequentialType>(*GTI)) {
+      if (ZExtInst *Extended = dyn_cast<ZExtInst>(*I)) {
+        unsigned SrcBitWidth =
+            cast<IntegerType>(Extended->getSrcTy())->getBitWidth();
+        // For GEP operand zext(a), if a <= max signed value of typeof(a), then
+        // the sign bit of a is zero and sext(a) = zext(a). Because the GEP is
+        // in bounds, we know a <= ObjectSize, so the condition can be reduced
+        // to ObjectSize <= max signed value of typeof(a).
+        if (ObjectSize <=
+            APInt::getSignedMaxValue(SrcBitWidth).getZExtValue()) {
+          *I = new SExtInst(Extended->getOperand(0), Extended->getType(),
+                            Extended->getName(), GEP);
+          Changed = true;
+        }
+      }
+    }
+  }
+  return Changed;
+}
+
 int64_t
 SeparateConstOffsetFromGEP::accumulateByteOffset(GetElementPtrInst *GEP,
                                                  bool &NeedsExtraction) {
@@ -631,6 +686,7 @@ bool SeparateConstOffsetFromGEP::splitGE
 
   bool Changed = false;
   Changed |= canonicalizeArrayIndicesToPointerSize(GEP);
+  Changed |= convertInBoundsZExtToSExt(GEP);
 
   bool NeedsExtraction;
   int64_t AccumulativeByteOffset = accumulateByteOffset(GEP, NeedsExtraction);

Modified: llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll?rev=210444&r1=210443&r2=210444&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll (original)
+++ llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll Sun Jun  8 18:49:34 2014
@@ -98,3 +98,44 @@ define void @sum_of_array2(i32 %x, i32 %
 ; IR: getelementptr float addrspace(3)* [[BASE_PTR]], i64 1
 ; IR: getelementptr float addrspace(3)* [[BASE_PTR]], i64 32
 ; IR: getelementptr float addrspace(3)* [[BASE_PTR]], i64 33
+
+; Similar to @sum_of_array3, but extends array indices using zext instead of
+; sext. e.g., array[zext(x + 1)][zext(y + 1)].
+define void @sum_of_array3(i32 %x, i32 %y, float* nocapture %output) {
+.preheader:
+  %0 = zext i32 %y to i64
+  %1 = zext i32 %x to i64
+  %2 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array, i64 0, i64 %1, i64 %0
+  %3 = addrspacecast float addrspace(3)* %2 to float*
+  %4 = load float* %3, align 4
+  %5 = fadd float %4, 0.000000e+00
+  %6 = add i32 %y, 1
+  %7 = zext i32 %6 to i64
+  %8 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array, i64 0, i64 %1, i64 %7
+  %9 = addrspacecast float addrspace(3)* %8 to float*
+  %10 = load float* %9, align 4
+  %11 = fadd float %5, %10
+  %12 = add i32 %x, 1
+  %13 = zext i32 %12 to i64
+  %14 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array, i64 0, i64 %13, i64 %0
+  %15 = addrspacecast float addrspace(3)* %14 to float*
+  %16 = load float* %15, align 4
+  %17 = fadd float %11, %16
+  %18 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array, i64 0, i64 %13, i64 %7
+  %19 = addrspacecast float addrspace(3)* %18 to float*
+  %20 = load float* %19, align 4
+  %21 = fadd float %17, %20
+  store float %21, float* %output, align 4
+  ret void
+}
+; PTX-LABEL: sum_of_array3(
+; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG:%(rl|r)[0-9]+]]{{\]}}
+; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG]]+4{{\]}}
+; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG]]+128{{\]}}
+; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG]]+132{{\]}}
+
+; IR-LABEL: @sum_of_array3(
+; IR: [[BASE_PTR:%[a-zA-Z0-9]+]] = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array, i64 0, i64 %{{[a-zA-Z0-9]+}}, i64 %{{[a-zA-Z0-9]+}}
+; IR: getelementptr float addrspace(3)* [[BASE_PTR]], i64 1
+; IR: getelementptr float addrspace(3)* [[BASE_PTR]], i64 32
+; IR: getelementptr float addrspace(3)* [[BASE_PTR]], i64 33

Modified: llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll?rev=210444&r1=210443&r2=210444&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll (original)
+++ llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll Sun Jun  8 18:49:34 2014
@@ -26,21 +26,23 @@ entry:
 ; CHECK-LABEL: @struct(
 ; CHECK: getelementptr [1024 x %struct.S]* @struct_array, i64 0, i64 %{{[a-zA-Z0-9]+}}, i32 1
 
-; We should be able to trace into s/zext(a + b) if a + b is non-negative
+; We should be able to trace into sext(a + b) if a + b is non-negative
 ; (e.g., used as an index of an inbounds GEP) and one of a and b is
 ; non-negative.
 define float* @sext_add(i32 %i, i32 %j) {
 entry:
   %0 = add i32 %i, 1
   %1 = sext i32 %0 to i64  ; inbound sext(i + 1) = sext(i) + 1
-  %2 = sub i32 %j, 2
-  ; However, inbound sext(j - 2) != sext(j) - 2, e.g., j = INT_MIN
+  %2 = add i32 %j, -2
+  ; However, inbound sext(j + -2) != sext(j) + -2, e.g., j = INT_MIN
   %3 = sext i32 %2 to i64
   %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64 0, i64 %1, i64 %3
   ret float* %p
 }
 ; CHECK-LABEL: @sext_add(
 ; CHECK-NOT: = add
+; CHECK: add i32 %j, -2
+; CHECK: sext
 ; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array, i64 0, i64 %{{[a-zA-Z0-9]+}}, i64 %{{[a-zA-Z0-9]+}}
 ; CHECK: getelementptr float* %{{[a-zA-Z0-9]+}}, i64 32
 
@@ -232,3 +234,28 @@ entry:
 ; CHECK-LABEL: @and(
 ; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array
 ; CHECK-NOT: getelementptr
+
+; if zext(a + b) <= max signed value of typeof(a + b), then we can prove
+; a + b >= 0 and zext(a + b) == sext(a + b). If we can prove further a or b is
+; non-negative, we have zext(a + b) == sext(a) + sext(b).
+define float* @inbounds_zext_add(i32 %i, i4 %j) {
+entry:
+  %0 = add i32 %i, 1
+  %1 = zext i32 %0 to i64
+  ; Because zext(i + 1) is an index of an in bounds GEP based on
+  ; float_2d_array, zext(i + 1) <= sizeof(float_2d_array) = 4096.
+  ; Furthermore, since typeof(i + 1) is i32 and 4096 < 2^31, we are sure the
+  ; sign bit of i + 1 is 0. This implies zext(i + 1) = sext(i + 1).
+  %2 = add i4 %j, 2
+  %3 = zext i4 %2 to i64
+  ; In this case, typeof(j + 2) is i4, so zext(j + 2) <= 4096 does not imply
+  ; the sign bit of j + 2 is 0.
+  %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64 0, i64 %1, i64 %3
+  ret float* %p
+}
+; CHECK-LABEL: @inbounds_zext_add(
+; CHECK-NOT: add
+; CHECK: add i4 %j, 2
+; CHECK: sext
+; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array, i64 0, i64 %{{[a-zA-Z0-9]+}}, i64 %{{[a-zA-Z0-9]+}}
+; CHECK: getelementptr float* %{{[a-zA-Z0-9]+}}, i64 32





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