[LLVMbugs] [Bug 17261] New: invoke of fcn ptr under -O0 isnt correctly restore spilled reg before callq

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Mon Sep 16 18:50:19 PDT 2013


            Bug ID: 17261
           Summary: invoke of fcn ptr under -O0 isnt correctly restore
                    spilled reg before callq
           Product: new-bugs
           Version: unspecified
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: pfllvmbugz at pnkfx.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 11216
  --> http://llvm.org/bugs/attachment.cgi?id=11216&action=edit
isolate.invoke.ll: LLVM input using invoke

This bug is an attempt to isolate the problem identified on the Rust project

Summary bullet points for reproduction:

  * It appears to be a problem with `invoke` reloading a spilled call target;
using `call` fixes the problem.

  * You need `llc -O0` to see the problem; higher optimization levels mask the

  * This bug report is written based on assembly code analysis, but the problem
was discovered (as you might expect) based on actual crashes of compiled code.

The plot:

I have found a standalone .ll input for which an `invoke` instruction compiles
via `llc` into code that loads from a location on the stack to which no value
has been previously assigned.  When I substitute a `call` instruction for the
same spot, `llc` generates code that correctly loads a spilled value from the
stack back into a register in preparation for the call.

I am running llc via command lines like this:
  llc -o isolate.invoke.ll.s isolate.invoke.ll -O0 -filetype=asm

In particular, to reproduce the bug, you need to use -O0.

I am running with `llc` compiled from:
  commit 107cfa2169299fe67caebe7d1f0d405ce727e420
  Author: Tim Northover <tnorthover at apple.com>
  Date:   Mon Sep 16 17:33:40 2013 +0000

      TableGen: fix constness of new comparison function.

      libc++ didn't seem to like a non-const call operator.

      git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190797

I have not been able to come up with a C++ program that generates the offending
code.  I am hoping that people will be able to see the same problem on their
own systems by inspecting the assembly (as I have been) and then be able to
figure out why `invoke` is not being compiled correctly.

I'm attaching four files: two are the input .ll files (one using `call`, and
the other using `invoke`), and the other two are the output .s files.

For ease of reference, I am posting the most relevant bits of generated
assembly here.

The variant that uses the call instruction generates correct machine code that
looks like this (snippet):

    ## BB#0:                                ## %function top level
    subq    $56, %rsp
    .cfi_def_cfa_offset 64
    addq    $8, %rdi
    movq    %rdi, 8(%rsp)           ## 8-byte Spill
    callq    __ZN9breakdown17haf444eb869dbfb724v0.0E
    movq    %rax, 32(%rsp)
    movq    32(%rsp), %rdi
    movq    8(%rsp), %rax           ## 8-byte Reload
    callq    *%rax

In particular, the spill and reload are both referring to the same stack
location, `8(%rsp)`

Conversely, the code that is generated from using `invoke` looks like this:

## BB#0:                                ## %function top level
    subq    $88, %rsp
    .cfi_def_cfa_offset 96
    addq    $8, %rdi
    movq    %rdi, 40(%rsp)          ## 8-byte Spill
    callq    __ZN9breakdown17haf444eb869dbfb724v0.0E
    movq    %rax, 32(%rsp)          ## 8-byte Spill
    jmp    LBB1_1
LBB1_1:                                 ## %normal return
    movq    32(%rsp), %rax          ## 8-byte Reload
    movq    %rax, 64(%rsp)
    movq    64(%rsp), %rdi
    movq    24(%rsp), %rcx          ## 8-byte Reload
    callq    *%rcx

Note that the reload that establishes %rcx is coming from `24(%rsp)`, a stack
location that has not been established by any register spills.

(Note: The attached code is only meant to illustrate incorrect assembly+machine
code generation; it is the result of me iteratively reducing an initial
function from the Rust bug linked above until I came up with something small
enough for reasonable submission, i.e. <50 lines.  But it does not run on its
own, and at this point I removed so much of the original computation that it
almost certainly would break if you did actually run it.)

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