AMDGPU: Allow negative MUBUF vaddr for gfx9

GFX9 does not enable bounds checking for the resource descriptors
used for private access, so it should be OK to use vaddr with
a potentially negative value.

llvm-svn: 319393

1 files changed, 15 insertions, 4 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
index 836c70609c0..79d8de92031 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
@@ -1158,14 +1158,25 @@ bool AMDGPUDAGToDAGISel::SelectMUBUFScratchOffen(SDNode *Parent,
     SDValue N0 = Addr.getOperand(0);
     SDValue N1 = Addr.getOperand(1);
 
-    // Offsets in vaddr must be positive.
+    // Offsets in vaddr must be positive if range checking is enabled.
     //
-    // The total computation of vaddr + soffset + offset must not overflow.
-    // If vaddr is negative, even if offset is 0 the sgpr offset add will end up
+    // The total computation of vaddr + soffset + offset must not overflow.  If
+    // vaddr is negative, even if offset is 0 the sgpr offset add will end up
     // overflowing.
+    //
+    // Prior to gfx9, MUBUF instructions with the vaddr offset enabled would
+    // always perform a range check. If a negative vaddr base index was used,
+    // this would fail the range check. The overall address computation would
+    // compute a valid address, but this doesn't happen due to the range
+    // check. For out-of-bounds MUBUF loads, a 0 is returned.
+    //
+    // Therefore it should be safe to fold any VGPR offset on gfx9 into the
+    // MUBUF vaddr, but not on older subtargets which can only do this if the
+    // sign bit is known 0.
     ConstantSDNode *C1 = cast<ConstantSDNode>(N1);
     if (SIInstrInfo::isLegalMUBUFImmOffset(C1->getZExtValue()) &&
-        CurDAG->SignBitIsZero(N0)) {
+        (!Subtarget->privateMemoryResourceIsRangeChecked() ||
+         CurDAG->SignBitIsZero(N0))) {
       std::tie(VAddr, SOffset) = foldFrameIndex(N0);
       ImmOffset = CurDAG->getTargetConstant(C1->getZExtValue(), DL, MVT::i16);
       return true;