[AMDGPU] Sink uniform buffer address offsets into soffset

This patch implements an optimization to partition MUBUF load/store offsets
into vector and scalar components for better address coalescing and reduced
VGPR pressure.

Transform buffer operations where voffset = add(uniform, divergent) by
moving the uniform part to soffset and keeping the divergent part in voffset.

Before:
  v_add_u32 v1, v0, sN
  buffer_{load,store}_T v*, v1, s[bufDesc:bufDesc+3] offen

After:
  buffer_{load,store}_T v*, v0, s[bufDesc:bufDesc+3], sN offen

The optimization currently applies to raw buffer loads/stores when soffset is
initially zero.

Tests includes comprehensive validation of both buffer loads and stores
across various supported variants (i8, i16, i32, vectors, floats) with
positive and negative test cases.

Author: PrasoonMishra

llvm/lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp

@@ -260,6 +260,7 @@ class AMDGPUCodeGenPrepareImpl
   bool visitIntrinsicInst(IntrinsicInst &I);
   bool visitFMinLike(IntrinsicInst &I);
   bool visitSqrt(IntrinsicInst &I);
+  bool visitBufferIntrinsic(IntrinsicInst &I);
   bool run();
 };
 
@@ -1910,6 +1911,15 @@ bool AMDGPUCodeGenPrepareImpl::visitIntrinsicInst(IntrinsicInst &I) {
     return visitFMinLike(I);
   case Intrinsic::sqrt:
     return visitSqrt(I);
+  case Intrinsic::amdgcn_raw_buffer_load:
+  case Intrinsic::amdgcn_raw_buffer_load_format:
+  case Intrinsic::amdgcn_raw_buffer_store:
+  case Intrinsic::amdgcn_raw_buffer_store_format:
+  case Intrinsic::amdgcn_raw_ptr_buffer_load:
+  case Intrinsic::amdgcn_raw_ptr_buffer_load_format:
+  case Intrinsic::amdgcn_raw_ptr_buffer_store:
+  case Intrinsic::amdgcn_raw_ptr_buffer_store_format:
+    return visitBufferIntrinsic(I);
   default:
     return false;
   }
@@ -2046,6 +2056,75 @@ bool AMDGPUCodeGenPrepareImpl::visitSqrt(IntrinsicInst &Sqrt) {
   return true;
 }
 
+/// Sink uniform addends in buffer address calculations into soffset.
+///
+/// Transforms buffer loads/stores with voffset = add(uniform, divergent)
+/// into voffset = divergent, soffset = uniform for better address coalescing
+/// Only applies to raw buffer operations with soffset initially zero.
+bool AMDGPUCodeGenPrepareImpl::visitBufferIntrinsic(IntrinsicInst &I) {
+  Intrinsic::ID IID = I.getIntrinsicID();
+  bool IsLoad = (IID == Intrinsic::amdgcn_raw_buffer_load ||
+                 IID == Intrinsic::amdgcn_raw_buffer_load_format ||
+                 IID == Intrinsic::amdgcn_raw_ptr_buffer_load ||
+                 IID == Intrinsic::amdgcn_raw_ptr_buffer_load_format);
+  bool IsStore = (IID == Intrinsic::amdgcn_raw_buffer_store ||
+                  IID == Intrinsic::amdgcn_raw_buffer_store_format ||
+                  IID == Intrinsic::amdgcn_raw_ptr_buffer_store ||
+                  IID == Intrinsic::amdgcn_raw_ptr_buffer_store_format);
+
+  if (!IsLoad && !IsStore)
+    return false;
+
+  // Buffer intrinsic operand layout (same for vector and pointer descriptor):
+  // Load:  (rsrc, voffset, soffset, cachepolicy)
+  // Store: (vdata, rsrc, voffset, soffset, cachepolicy)
+  const unsigned VOffsetIdx = IsStore ? 2 : 1;
+  const unsigned SOffsetIdx = IsStore ? 3 : 2;
+
+  Value *VOffset = I.getArgOperand(VOffsetIdx);
+  Value *SOffset = I.getArgOperand(SOffsetIdx);
+
+  // Only optimize when soffset is currently zero
+  if (!match(SOffset, m_Zero()))
+    return false;
+
+  // Pattern match: voffset = add(uniform, divergent)
+  Value *LHS, *RHS;
+  if (!match(VOffset, m_Add(m_Value(LHS), m_Value(RHS))))
+    return false;
+
+  bool LHSUniform = UA.isUniform(LHS);
+  bool RHSUniform = UA.isUniform(RHS);
+
+  // Need exactly one uniform and one divergent operand.
+  // TODO: Handle the case where both are uniform.
+  if (LHSUniform == RHSUniform)
+    return false;
+
+  Value *UniformAddend = LHSUniform ? LHS : RHS;
+  Value *DivergentAddend = LHSUniform ? RHS : LHS;
+
+  // Skip if the uniform addend is a non-negative constant that fits in the
+  // 12-bit immediate offset field. The backend will fold it into the immediate
+  // field, which avoids consuming an soffset operand.
+  // Negative or large constants must use soffset.
+  if (auto *CI = dyn_cast<ConstantInt>(UniformAddend)) {
+    int64_t Offset = CI->getSExtValue();
+    if (Offset >= 0 && Offset <= 4095)
+      return false;
+  }
+
+  LLVM_DEBUG(dbgs() << "AMDGPUCodeGenPrepare: Sinking uniform addend into "
+                       "soffset for buffer "
+                    << (IsStore ? "store" : "load") << ": " << I << '\n');
+
+  // Update voffset and soffset operands
+  I.setArgOperand(VOffsetIdx, DivergentAddend);
+  I.setArgOperand(SOffsetIdx, UniformAddend);
+
+  return true;
+}
+
 bool AMDGPUCodeGenPrepare::runOnFunction(Function &F) {
   if (skipFunction(F))
     return false;