[AMDGPU] Let LowerModuleLDS run twice on the same module (#81729)

If all variables in the module are absolute, this means we're running
the pass again on an already lowered module, and that works.
If none of them are absolute, lowering can proceed as usual.
Only diagnose cases where we have a mix of absolute/non-absolute GVs,
which means we added LDS GVs after lowering, which is broken.

See #81491
Split from #75333

Author: Pierre-vh

llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp

@@ -340,15 +340,25 @@ class AMDGPULowerModuleLDS {
 
     // Get uses from the current function, excluding uses by called functions
     // Two output variables to avoid walking the globals list twice
+    std::optional<bool> HasAbsoluteGVs;
     for (auto &GV : M.globals()) {
       if (!AMDGPU::isLDSVariableToLower(GV)) {
         continue;
       }
 
-      if (GV.isAbsoluteSymbolRef()) {
-        report_fatal_error(
-            "LDS variables with absolute addresses are unimplemented.");
-      }
+      // Check if the module is consistent: either all GVs are absolute (happens
+      // when we run the pass more than once), or none are.
+      const bool IsAbsolute = GV.isAbsoluteSymbolRef();
+      if (HasAbsoluteGVs.has_value()) {
+        if (*HasAbsoluteGVs != IsAbsolute) {
+          report_fatal_error(
+              "Module cannot mix absolute and non-absolute LDS GVs");
+        }
+      } else
+        HasAbsoluteGVs = IsAbsolute;
+
+      if (IsAbsolute)
+        continue;
 
       for (User *V : GV.users()) {
         if (auto *I = dyn_cast<Instruction>(V)) {

llvm/test/CodeGen/AMDGPU/lds-reject-absolute-addresses.ll renamed to llvm/test/CodeGen/AMDGPU/lds-reject-mixed-absolute-addresses.ll

@@ -2,8 +2,9 @@
 ; RUN: not --crash opt -S -mtriple=amdgcn-- -passes=amdgpu-lower-module-lds < %s 2>&1 | FileCheck %s
 
 @var1 = addrspace(3) global i32 undef, !absolute_symbol !0
+@var2 = addrspace(3) global i32 undef
 
-; CHECK: LLVM ERROR: LDS variables with absolute addresses are unimplemented.
+; CHECK: Module cannot mix absolute and non-absolute LDS GVs
 define amdgpu_kernel void @kern() {
   %val0 = load i32, ptr addrspace(3) @var1
   %val1 = add i32 %val0, 4
@@ -12,4 +13,3 @@ define amdgpu_kernel void @kern() {
 }
 
 !0 = !{i32 0, i32 1}
-

llvm/test/CodeGen/AMDGPU/lds-run-twice-absolute-md.ll

@@ -0,0 +1,16 @@
+; RUN: opt -S -mtriple=amdgcn-- -amdgpu-lower-module-lds %s -o %t.ll
+; RUN: opt -S -mtriple=amdgcn-- -amdgpu-lower-module-lds %t.ll -o %t.second.ll
+; RUN: diff -ub %t.ll %t.second.ll -I ".*ModuleID.*"
+
+; Check AMDGPULowerModuleLDS can run more than once on the same module, and that
+; the second run is a no-op.
+
+@lds = internal unnamed_addr addrspace(3) global i32 undef, align 4, !absolute_symbol !0
+
+define amdgpu_kernel void @test() {
+entry:
+  store i32 1, ptr addrspace(3) @lds
+  ret void
+}
+
+!0 = !{i32 0, i32 1}

llvm/test/CodeGen/AMDGPU/lds-run-twice.ll

@@ -0,0 +1,14 @@
+; RUN: opt -S -mtriple=amdgcn-- -amdgpu-lower-module-lds %s -o %t.ll
+; RUN: opt -S -mtriple=amdgcn-- -amdgpu-lower-module-lds %t.ll -o %t.second.ll
+; RUN: diff -ub %t.ll %t.second.ll -I ".*ModuleID.*"
+
+; Check AMDGPULowerModuleLDS can run more than once on the same module, and that
+; the second run is a no-op.
+
+@lds = internal unnamed_addr addrspace(3) global i32 undef, align 4
+
+define amdgpu_kernel void @test() {
+entry:
+  store i32 1, ptr addrspace(3) @lds
+  ret void
+}