[all-commits] [llvm/llvm-project] c11627: [MLIR][LLVM] Fix memory explosion when converting ...

[Xiang Li via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c11627c2f4d550613a3cb360c89a0cf52d2eb720
      https://github.com/llvm/llvm-project/commit/c11627c2f4d550613a3cb360c89a0cf52d2eb720
  Author: Xiang Li <python3kgae at outlook.com>
  Date:   2024-02-26 (Mon, 26 Feb 2024)

  Changed paths:
    M mlir/lib/Target/LLVMIR/ModuleTranslation.cpp
    A mlir/test/Target/LLVMIR/erase-dangling-constants.mlir

  Log Message:
  -----------
  [MLIR][LLVM] Fix memory explosion when converting global variable bodies in ModuleTranslation (#82708)

There is memory explosion when converting the body or initializer region
of a large global variable, e.g. a constant array.

For example, when translating a constant array of 100000 strings:

llvm.mlir.global internal constant @cats_strings() {addr_space = 0 :
i32, alignment = 16 : i64} : !llvm.array<100000 x ptr<i8>> {
    %0 = llvm.mlir.undef : !llvm.array<100000 x ptr<i8>>
    %1 = llvm.mlir.addressof @om_1 : !llvm.ptr<array<1 x i8>>
%2 = llvm.getelementptr %1[0, 0] : (!llvm.ptr<array<1 x i8>>) ->
!llvm.ptr<i8>
    %3 = llvm.insertvalue %2, %0[0] : !llvm.array<100000 x ptr<i8>>
    %4 = llvm.mlir.addressof @om_2 : !llvm.ptr<array<1 x i8>>
%5 = llvm.getelementptr %4[0, 0] : (!llvm.ptr<array<1 x i8>>) ->
!llvm.ptr<i8>
    %6 = llvm.insertvalue %5, %3[1] : !llvm.array<100000 x ptr<i8>>
    %7 = llvm.mlir.addressof @om_3 : !llvm.ptr<array<1 x i8>>
%8 = llvm.getelementptr %7[0, 0] : (!llvm.ptr<array<1 x i8>>) ->
!llvm.ptr<i8>
    %9 = llvm.insertvalue %8, %6[2] : !llvm.array<100000 x ptr<i8>>
    %10 = llvm.mlir.addressof @om_4 : !llvm.ptr<array<1 x i8>>
%11 = llvm.getelementptr %10[0, 0] : (!llvm.ptr<array<1 x i8>>) ->
!llvm.ptr<i8>
    %12 = llvm.insertvalue %11, %9[3] : !llvm.array<100000 x ptr<i8>>

    ... (ignore the remaining part)
}

where @om_1, @om_2, ... are string global constants.

Each time an operation is converted to LLVM, a new constant is created.
When it comes to llvm.insertvalue, a new constant array of 100000
elements is created and the old constant array (input) is not destroyed.
This causes memory explosion. We observed that, on a system with 128 GB
memory, the translation of 100000 elements got killed due to using up
all the memory. On a system with 64 GB, 65536 elements was enough to
cause the translation killed.

There is a previous patch (https://reviews.llvm.org/D148487) which fix
this issue but was reverted for
https://github.com/llvm/llvm-project/issues/62802

The old patch checks generated constants and destroyed them if there is
no use. But the check of use for the constant is too early, which cause
the constant be removed before use.


This new patch added a map was added a map to save expected use count
for a constant. Then decrease when reach each use.
And only erase the constant when the use count reach to zero

With new patch, the repro in
https://github.com/llvm/llvm-project/issues/62802 finished correctly.



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