[LLVMbugs] [Bug 19972] New: Bad code generation in post-ra ARMLoadStoreOptimizer pass in thumb1 mode

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19972

            Bug ID: 19972
           Summary: Bad code generation in post-ra ARMLoadStoreOptimizer
                    pass in thumb1 mode
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: sgundapa at codeaurora.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 12624
  --> http://llvm.org/bugs/attachment.cgi?id=12624&action=edit
Test case to demonstrate the bad codegen

Hi,
   Please find the attached test case which can demonstrate the bad code
generation in  post-ra ARMLoadStoreOptimizer pass in thumb1 mode.

If I compile with tip, I get this assembly:
  $ clang -target arm  -Os -mthumb  -mcpu=arm926ej-s -c  t.c -S -o-
  @ BB#0:                                 @ %entry
          push    {r7, lr}
          add     r7, sp, #0
          subs    r0, #8
          ldm     r0!, {r1, r2}
          bl      bar
          pop     {r7, pc}

The base address is incorrectly decremented before the load, so we end up
loading the incorrect values.

If I disable the post-ra ARMLoadStoreOptimizer pass, I get this assembly:
  @ BB#0:                                 @ %entry
          push    {r7, lr}
          add     r7, sp, #0
          ldr     r1, [r0]
          ldr     r2, [r0, #4]
          bl      bar
          pop     {r7, pc}

which correctly loads the values.

I would expect the ARMLoadStoreOptimizer to generate assembly like this for the
source code:
          push    {r7, lr}
          add     r7, sp, #0
          ldm     r0, {r1, r2}
          bl      bar
          pop     {r7, pc}

I triage the commit which initially caused the bad codegen.
The commit is 

commit 40ae57cc0a187424c8db723a51506b7192b77f71
Author: James Molloy <james.molloy at arm.com>
Date:   Fri May 16 14:14:30 2014 +0000

    Fix the Load/Store optimization pass to work with Thumb1.
    Patch by Moritz Roth!
    git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208992
91177308-0d34-0410-b5e6-96231b3b80d8

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20140606/c96bb1c3/attachment.html>