[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

1 files changed, 14 insertions, 4 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp b/llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp
index 5762f19..b85cb26 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp
@@ -340,15 +340,25 @@ public:
 
     // 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)) {