[flang][NFC] turn (h)fir.declare side effect into debug ressource alloca

[jeanPerier]

See https://reviews.llvm.org/D157626 for the rational of declare having side effects.

The write effect is to scary for passes that look for read/write effects without caring about the resource affected. I know Slava asked for it, but I think the creation of the DebuggingResource was enough and that a write is too much. The alloca effect is sufficient to prevent DCE to remove it, which is all we care about currently.

This currently is flag as a reason for creating LHS temporary in assignment to vector subscripted entity with array constructor.
There is a lot of read/write side effect analysis in the "lower-hlfir-ordered-assignments" pass, and I feel like we will just keep adding weird "debug ressource" bypassing here and there with these side effects.

[llvmbot]

@llvm/pr-subscribers-flang-fir-hlfir

Author: None (jeanPerier)

Changes

See https://reviews.llvm.org/D157626 for the rational of declare having side effects.

The write effect is to scary for passes that look for read/write effects without caring about the resource affected. I know Slava asked for it, but I think the creation of the DebuggingResource was enough and that a write is too much. The alloca effect is sufficient to prevent DCE to remove it, which is all we care about currently.

This currently is flag as a reason for creating LHS temporary in assignment to vector subscripted entity with array constructor.
There is a lot of read/write side effect analysis in the "lower-hlfir-ordered-assignments" pass, and I feel like we will just keep adding weird "debug ressource" bypassing here and there with these side effects.

---

Full diff: https://github.com/llvm/llvm-project/pull/113321.diff

2 Files Affected:

• (modified) flang/include/flang/Optimizer/Dialect/FIROps.td (+1-1)
• (modified) flang/include/flang/Optimizer/HLFIR/HLFIROps.td (+1-1)

diff --git a/flang/include/flang/Optimizer/Dialect/FIROps.td b/flang/include/flang/Optimizer/Dialect/FIROps.td
index d467f0ab4e1c1e..eda8f26e936fb6 100644
--- a/flang/include/flang/Optimizer/Dialect/FIROps.td
+++ b/flang/include/flang/Optimizer/Dialect/FIROps.td
@@ -3078,7 +3078,7 @@ def fir_IsPresentOp : fir_SimpleOp<"is_present", [NoMemoryEffect]> {
 // debug information so we would like to keep this around even if the value
 // is not used.
 def fir_DeclareOp : fir_Op<"declare", [AttrSizedOperandSegments,
-    MemoryEffects<[MemWrite<DebuggingResource>]>,
+    MemoryEffects<[MemAlloc<DebuggingResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable";
 
diff --git a/flang/include/flang/Optimizer/HLFIR/HLFIROps.td b/flang/include/flang/Optimizer/HLFIR/HLFIROps.td
index fdf0db9d3c75de..354d3baf8c2067 100644
--- a/flang/include/flang/Optimizer/HLFIR/HLFIROps.td
+++ b/flang/include/flang/Optimizer/HLFIR/HLFIROps.td
@@ -36,7 +36,7 @@ class hlfir_Op<string mnemonic, list<Trait> traits>
 // from the declare operation can be used to generate debug information so we
 // don't want to remove it as dead code
 def hlfir_DeclareOp : hlfir_Op<"declare", [AttrSizedOperandSegments,
-    MemoryEffects<[MemWrite<DebuggingResource>]>,
+    MemoryEffects<[MemAlloc<DebuggingResource>]>,
     DeclareOpInterfaceMethods<fir_FortranVariableOpInterface>]> {
   let summary = "declare a variable and produce an SSA value that can be used as a variable in HLFIR operations";
 

[tblah]

LGTM, thanks!