Update comment slightlyusers/ruiling/opt-global-addr

2 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
index ff3eb0e1eee0..1064df8a9080 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
@@ -2053,8 +2053,9 @@ bool AMDGPUDAGToDAGISel::SelectGlobalSAddr(SDNode *N, SDValue Addr,
           if (TII->isLegalFLATOffset(COffset, AMDGPUAS::GLOBAL_ADDRESS,
                                      SIInstrFlags::FlatGlobal)) {
             // If the MSB of the first operand of the addition is known to be
-            // zero, which is followed by zext, we are sure overflow would not
-            // happen during addition.
+            // zero, which is followed by zext, this makes sure:
+            // a.) for unsigned VOffset, no overflow would happen.
+            // b.) for signed VOffset, the value can be properly encoded.
             if (RHS.getOpcode() == ISD::ZERO_EXTEND &&
                 CurDAG->SignBitIsZero(ExtRHS.getOperand(0))) {
               VOffset = ExtRHS.getOperand(0);
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp b/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
index 44eacf6e4ed4..0a29cdd4cbda 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
@@ -5856,8 +5856,9 @@ AMDGPUInstructionSelector::selectGlobalSAddr(MachineOperand &Root,
                                       AMDGPUAS::GLOBAL_ADDRESS,
                                       SIInstrFlags::FlatGlobal)) {
               // If the MSB of the first operand of the addition is known to be
-              // zero, which is followed by zext, we are sure overflow would not
-              // happen for the addition.
+              // zero, which is followed by zext, this makes sure:
+              // a.) for unsigned VOffset, no overflow would happen.
+              // b.) for signed VOffset, the value can be properly encoded.
               if (matchZeroExtendFromS32OrS32(PtrBaseOffset) &&
                   VT->signBitIsZero(VOffsetDef->getOperand(1).getReg())) {
                 VOffset = VOffsetDef->getOperand(1).getReg();