[AMDGPU][LowerModuleLDS] Refactor partially lowered module detection (llvm#85793)

Refactor the logic that checks if a module contains mixed
absolute/non-lowered LDS GVs.

The check now happens latter when the "worklists" are formed. This is
because in some cases (OpenMP) we can have non-lowered GVs in a lowered
module, and this is normal because those GVs are just unused and removed
from the list at some point before the end of `getUsesOfLDSByFunction`.

Doing the check later ensures that if a mixed module is spotted, then
it's a _real_ mixed module that needs rejection, not a module containing
an intentionally ignored GV.

Author: Pierre-vh

llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp

@@ -340,26 +340,11 @@ 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;
       }
 
-      // 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)) {
           Function *F = I->getFunction();
@@ -469,6 +454,31 @@ class AMDGPULowerModuleLDS {
       }
     }
 
+    // Verify that we fall into one of 2 cases:
+    //    - All variables are absolute: this is a re-run of the pass
+    //      so we don't have anything to do.
+    //    - No variables are absolute.
+    std::optional<bool> HasAbsoluteGVs;
+    for (auto &Map : {direct_map_kernel, indirect_map_kernel}) {
+      for (auto &[Fn, GVs] : Map) {
+        for (auto *GV : GVs) {
+          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 we only had absolute GVs, we have nothing to do, return an empty
+    // result.
+    if (HasAbsoluteGVs && *HasAbsoluteGVs)
+      return {FunctionVariableMap(), FunctionVariableMap()};
+
     return {std::move(direct_map_kernel), std::move(indirect_map_kernel)};
   }
 

llvm/test/CodeGen/AMDGPU/lds-mixed-absolute-addresses-unused.ll

@@ -0,0 +1,26 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -mtriple=amdgcn-- -amdgpu-lower-module-lds < %s 2>&1 | FileCheck %s
+; RUN: opt -S -mtriple=amdgcn-- -passes=amdgpu-lower-module-lds < %s 2>&1 | FileCheck %s
+
+; This looks like a partially lowered module, but the non-lowered GV isn't used by any kernels.
+; In such cases, LowerModuleLDS is free to leave it in and ignore it, and we want to make sure
+; LowerModuleLDS doesn't crash if it re-runs on such modules.
+@notLowered = addrspace(3) global i32 poison
+@lowered = addrspace(3) global i32 poison, !absolute_symbol !0
+
+@llvm.compiler.used = appending addrspace(1) global [1 x ptr] [ptr addrspacecast (ptr addrspace(3) @notLowered to ptr)], section "llvm.metadata"
+
+define amdgpu_kernel void @kern(i32 %val0) {
+; CHECK-LABEL: define amdgpu_kernel void @kern(
+; CHECK-SAME: i32 [[VAL0:%.*]]) {
+; CHECK-NEXT:    [[VAL1:%.*]] = add i32 [[VAL0]], 4
+; CHECK-NEXT:    store i32 [[VAL1]], ptr addrspace(3) @lowered, align 4
+; CHECK-NEXT:    ret void
+;
+  %val1 = add i32 %val0, 4
+  store i32 %val1, ptr addrspace(3) @lowered
+  ret void
+}
+
+
+!0 = !{i32 0, i32 1}

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

@@ -8,7 +8,7 @@
 define amdgpu_kernel void @kern() {
   %val0 = load i32, ptr addrspace(3) @var1
   %val1 = add i32 %val0, 4
-  store i32 %val1, ptr addrspace(3) @var1
+  store i32 %val1, ptr addrspace(3) @var2
   ret void
 }