[PATCH] optimize vector fabs of bitcasted constant integer values

[Sanjay Patel]

Hi rengolin, pete, chandlerc,

This patch allows vector fabs operations of bitcasted constant integer values to be optimized in the same way that we already optimize scalar fabs. 

So for code like this:
%bitcast = bitcast i64 18446744069414584320 to <2 x float> ; 0xFFFF_FFFF_0000_0000
%fabs = call <2 x float> @llvm.fabs.v2f32(<2 x float> %bitcast)
%ret = bitcast <2 x float> %fabs to i64

Instead of generating something like this:
    movabsq (constant pool loadi of mask for sign bits) 
    vmovq   (move from integer register to vector/fp register)
    vandps  (mask off sign bits)
    vmovq   (move vector/fp register back to integer return register)

 We should generate:
    mov     (put constant value in return register)


I have also removed a redundant clause in the first 'if' statement:
N0.getOperand(0).getValueType().isInteger() 

is the same thing as:
IntVT.isInteger() 

For more background, please see:
http://reviews.llvm.org/D4770

And:
http://llvm.org/bugs/show_bug.cgi?id=20354

http://reviews.llvm.org/D4785

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/vec_fabs.ll

Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7405,20 +7405,26 @@
 
   // Transform fabs(bitconvert(x)) -> bitconvert(x&~sign) to avoid loading
   // constant pool values.
-  // TODO: We can also optimize for vectors here, but we need to make sure
-  // that the sign mask is created properly for each vector element.
   if (!TLI.isFAbsFree(VT) &&
-      N0.getOpcode() == ISD::BITCAST && N0.getNode()->hasOneUse() &&
-      N0.getOperand(0).getValueType().isInteger() &&
-      !VT.isVector()) {
+      N0.getOpcode() == ISD::BITCAST &&
+      N0.getNode()->hasOneUse()) {
     SDValue Int = N0.getOperand(0);
     EVT IntVT = Int.getValueType();
     if (IntVT.isInteger() && !IntVT.isVector()) {
+      APInt SignMask;
+      if (N0.getValueType().isVector()) {
+        // For a vector, get a mask such as 0x7f... per scalar element
+        // and splat it.
+        SignMask = ~APInt::getSignBit(N0.getValueType().getScalarSizeInBits());
+        SignMask = APInt::getSplat(IntVT.getSizeInBits(), SignMask);
+      } else {
+        // For a scalar, just generate 0x7f...
+        SignMask = ~APInt::getSignBit(IntVT.getSizeInBits());
+      }
       Int = DAG.getNode(ISD::AND, SDLoc(N0), IntVT, Int,
-             DAG.getConstant(~APInt::getSignBit(IntVT.getSizeInBits()), IntVT));
+                        DAG.getConstant(SignMask, IntVT));
       AddToWorklist(Int.getNode());
-      return DAG.getNode(ISD::BITCAST, SDLoc(N),
-                         N->getValueType(0), Int);
+      return DAG.getNode(ISD::BITCAST, SDLoc(N), N->getValueType(0), Int);
     }
   }
 
Index: test/CodeGen/X86/vec_fabs.ll
===================================================================
--- test/CodeGen/X86/vec_fabs.ll
+++ test/CodeGen/X86/vec_fabs.ll
@@ -38,21 +38,38 @@
 declare <8 x float> @llvm.fabs.v8f32(<8 x float> %p)
 
 ; PR20354: when generating code for a vector fabs op,
-; make sure the correct mask is used for all vector elements.
-; CHECK-LABEL: .LCPI4_0:
-; CHECK-NEXT:    .long	2147483647
-; CHECK-NEXT:    .long	2147483647
-define i64 @fabs_v2f32(<2 x float> %v) {
-; CHECK-LABEL: fabs_v2f32:
-; CHECK:         movabsq $-9223372034707292160, %[[R:r[^ ]+]]
-; CHECK-NEXT:    vmovq %[[R]], %[[X:xmm[0-9]+]]
-; CHECK-NEXT:    vandps   {{.*}}.LCPI4_0{{.*}}, %[[X]], %[[X]]
-; CHECK-NEXT:    vmovq   %[[X]], %rax
-; CHECK-NEXT:    retq
-  %highbits = bitcast i64 9223372039002259456 to <2 x float> ; 0x8000_0000_8000_0000
-  %fabs = call <2 x float> @llvm.fabs.v2f32(<2 x float> %highbits)
-  %ret = bitcast <2 x float> %fabs to i64
-  ret i64 %ret
+; make sure that we're only turning off the sign bit of each float value.
+; No constant pool loads or vector ops are needed for the fabs of a
+; bitcasted integer constant; we should just return an integer constant
+; that has the sign bits turned off.
+;
+; So instead of something like this:
+;    movabsq (constant pool loadi of mask for sign bits) 
+;    vmovq   (move from integer register to vector/fp register)
+;    vandps  (mask off sign bits)
+;    vmovq   (move vector/fp register back to integer return register)
+;
+; We should generate:
+;    mov     (put constant value in return register)
+
+; CHECK-LABEL: fabs_v2f32_1
+define i64 @fabs_v2f32_1() {
+ %bitcast = bitcast i64 18446744069414584320 to <2 x float> ; 0xFFFF_FFFF_0000_0000
+ %fabs = call <2 x float> @llvm.fabs.v2f32(<2 x float> %bitcast)
+ %ret = bitcast <2 x float> %fabs to i64
+ ret i64 %ret
+; CHECK: movabsq $9223372032559808512, %rax
+;  # imm = 0x7FFF_FFFF_0000_0000
 }
 
+; CHECK-LABEL: fabs_v2f32_2
+define i64 @fabs_v2f32_2() {
+ %bitcast = bitcast i64 4294967295 to <2 x float> ; 0x0000_0000_FFFF_FFFF
+ %fabs = call <2 x float> @llvm.fabs.v2f32(<2 x float> %bitcast)
+ %ret = bitcast <2 x float> %fabs to i64
+ ret i64 %ret
+; CHECK: movl $2147483647, %eax
+;  # imm = 0x0000_0000_7FFF_FFFF
+}
+
 declare <2 x float> @llvm.fabs.v2f32(<2 x float> %p)
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