[Flang] Hoisting constant-sized allocas at flang codegen. (#95310)

This change modifies the `AllocaOpConversion` in flang codegen to insert
constant-sized LLVM allocas at the entry block of `LLVMFuncOp` or
OpenACC/OpenMP Op, rather than in-place at the `fir.alloca`. This
effectively hoists constant-sized FIR allocas to the proper block.

When compiling the example subroutine below with `flang-new`, we get a
llvm.stacksave/stackrestore pair around a constant-sized `fir.alloca
i32`.

```
subroutine test(n)
    block
      integer :: n
      print *, n
    end block
  end subroutine test
```

Without the proposed change, downstream LLVM compilation cannot hoist
this constant-sized alloca out of the stacksave/stackrestore region
which may lead to missed downstream optimizations:

```
*** IR Dump After Safe Stack instrumentation pass (safe-stack) ***
define void @test_(ptr %0) !dbg !3 {
  %2 = call ptr @llvm.stacksave.p0(), !dbg !7
  %3 = alloca i32, i64 1, align 4, !dbg !8
  %4 = call ptr @_FortranAioBeginExternalListOutput(i32 6, ptr @_QQclX62c91d05f046c7a656e7978eb13f2821, i32 4), !dbg !9
  %5 = load i32, ptr %3, align 4, !dbg !10, !tbaa !11
  %6 = call i1 @_FortranAioOutputInteger32(ptr %4, i32 %5), !dbg !10
  %7 = call i32 @_FortranAioEndIoStatement(ptr %4), !dbg !9
  call void @llvm.stackrestore.p0(ptr %2), !dbg !15
  ret void, !dbg !16
}
```

With this change, the `llvm.alloca` is already hoisted out of the
stacksave/stackrestore region during flang codegen:

```
// -----// IR Dump After FIRToLLVMLowering (fir-to-llvm-ir) //----- //
  llvm.func @test_(%arg0: !llvm.ptr {fir.bindc_name = "n"}) attributes {fir.internal_name = "_QPtest"} {
    %0 = llvm.mlir.constant(4 : i32) : i32
    %1 = llvm.mlir.constant(1 : i64) : i64
    %2 = llvm.alloca %1 x i32 {bindc_name = "n"} : (i64) -> !llvm.ptr
    %3 = llvm.mlir.constant(6 : i32) : i32
    %4 = llvm.mlir.undef : i1
    %5 = llvm.call @llvm.stacksave.p0() {fastmathFlags = #llvm.fastmath<contract>} : () -> !llvm.ptr
    %6 = llvm.mlir.addressof @_QQclX62c91d05f046c7a656e7978eb13f2821 : !llvm.ptr
    %7 = llvm.call @_FortranAioBeginExternalListOutput(%3, %6, %0) {fastmathFlags = #llvm.fastmath<contract>} : (i32, !llvm.ptr, i32) -> !llvm.ptr
    %8 = llvm.load %2 {tbaa = [#tbaa_tag]} : !llvm.ptr -> i32
    %9 = llvm.call @_FortranAioOutputInteger32(%7, %8) {fastmathFlags = #llvm.fastmath<contract>} : (!llvm.ptr, i32) -> i1
    %10 = llvm.call @_FortranAioEndIoStatement(%7) {fastmathFlags = #llvm.fastmath<contract>} : (!llvm.ptr) -> i32
    llvm.call @llvm.stackrestore.p0(%5) {fastmathFlags = #llvm.fastmath<contract>} : (!llvm.ptr) -> ()
    llvm.return
  }
```

---------

Co-authored-by: Vijay Kandiah <vkandiah@sky6.pgi.net>

0 files changed, 0 insertions, 0 deletions