[llvm] r215685 - [PPC64] Add missing dependency on X2 to LDinto_toc.

[Bill Schmidt]

On Fri, 2014-08-15 at 01:25 +0000, Bill Schmidt wrote:
> Author: wschmidt
> Date: Thu Aug 14 20:25:26 2014
> New Revision: 215685
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=215685&view=rev
> Log:
> [PPC64] Add missing dependency on X2 to LDinto_toc.
> 
> The LDinto_toc pattern has been part of 64-bit PowerPC for a long
> time, and represents loading from a memory location into the TOC
> register (X2).  However, this pattern doesn't explicitly record that
> it modifies that register.  This patch adds the missing dependency.
> 
> It was very surprising to me that this has never shown up as a problem
> in the past, and that we only saw this problem recently in a single
> scenario when building a self-hosted clang.  It turns out that in most
> cases we have another dependency present that keeps the LDinto_toc
> instruction tied in place.  LDinto_toc is used for TOC restore
> following a call site, so this is a typical sequence:
> 
>    BCTRL8 <regmask>, %CTR8<imp-use>, %RM<imp-use>, %X3<imp-use>, %X12<imp-use>, %X1<imp-def>, ...
>    LDinto_toc 24, %X1
>    ADJCALLSTACKUP 96, 0, %R1<imp-def>, %R1<imp-use>
> 
> Because the LDinto_toc is inserted prior to the ADJCALLSTACKUP, there
> is a natural anti-dependency between the two that keeps it in place.
> 
> Therefore we don't usually see a problem.  However, in one particular
> case, one call is followed immediately by another call, and the second
> call requires a parameter that is a TOC-relative address.  This is the
> code sequence:
> 
>   BCTRL8 <regmask>, %CTR8<imp-use>, %RM<imp-use>, %X3<imp-use>, %X4<imp-use>, %X5<imp-use>, %X12<imp-use>, %X1<imp-def>, ...
>   LDinto_toc 24, %X1
>   ADJCALLSTACKUP 96, 0, %R1<imp-def>, %R1<imp-use>
>   ADJCALLSTACKDOWN 96, %R1<imp-def>, %R1<imp-use>
>   %vreg39<def> = ADDIStocHA %X2, <ga:@.str>; G8RC_and_G8RC_NOX0:%vreg39
>   %vreg40<def> = ADDItocL %vreg39<kill>, <ga:@.str>; G8RC:%vreg40 G8RC_and_G8RC_NOX0:%vreg39
> 
> Note that the back-to-back stack adjustments are the same size!  The
> back end is smart enough to recognize this and optimize them away:
> 
>   BCTRL8 <regmask>, %CTR8<imp-use>, %RM<imp-use>, %X3<imp-use>, %X4<imp-use>, %X5<imp-use>, %X12<imp-use>, %X1<imp-def>, ...
>   LDinto_toc 24, %X1
>   %vreg39<def> = ADDIStocHA %X2, <ga:@.str>; G8RC_and_G8RC_NOX0:%vreg39
>   %vreg40<def> = ADDItocL %vreg39<kill>, <ga:@.str>; G8RC:%vreg40 G8RC_and_G8RC_NOX0:%vreg39
> 
> Now there is nothing to prevent the ADDIStocHA instruction from moving
> ahead of the LDinto_toc instruction, and because of the longest-path
> heuristic, this is what happens.
> 
> With the accompanying patch, %X2 is represented as an implicit def:
> 
>   BCTRL8 <regmask>, %CTR8<imp-use>, %RM<imp-use>, %X3<imp-use>, %X4<imp-use>, %X5<imp-use>, %X12<imp-use>, %X1<imp-def>, ...
>   LDinto_toc 24, %X1, %X2<imp-def,dead>
>   ADJCALLSTACKUP 96, 0, %R1<imp-def,dead>, %R1<imp-use>
>   ADJCALLSTACKDOWN 96, %R1<imp-def,dead>, %R1<imp-use>
>   %vreg39<def> = ADDIStocHA %X2, <ga:@.str>; G8RC_and_G8RC_NOX0:%vreg39
>   %vreg40<def> = ADDItocL %vreg39<kill>, <ga:@.str>; G8RC:%vreg40 G8RC_and_G8RC_NOX0:%vreg39
> 
> So now when the two stack adjustments are removed, ADDIStocHA is
> prevented from being moved above LDinto_toc.
> 
> I have not yet created a test case for this, because the original
> failure occurs on a relatively large function that needs reduction.
> However, this is a fairly serious bug, despite its infrequency, and I
> wanted to get this patch onto the list as soon as possible so that it
> can be considered for a 3.5 backport.  I'll work on whittling down a
> test case.

Test case added in r215711.

Thanks,
Bill

> 
> Have we missed the boat for 3.5 at this point?
> 
> Thanks,
> Bill
> 
> Modified:
>     llvm/trunk/lib/Target/PowerPC/PPCInstr64Bit.td
> 
> Modified: llvm/trunk/lib/Target/PowerPC/PPCInstr64Bit.td
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCInstr64Bit.td?rev=215685&r1=215684&r2=215685&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/PowerPC/PPCInstr64Bit.td (original)
> +++ llvm/trunk/lib/Target/PowerPC/PPCInstr64Bit.td Thu Aug 14 20:25:26 2014
> @@ -802,7 +802,7 @@ def LDtocCPT: Pseudo<(outs g8rc:$rD), (i
>                    [(set i64:$rD,
>                       (PPCtoc_entry tconstpool:$disp, i64:$reg))]>, isPPC64;
> 
> -let hasSideEffects = 1, isCodeGenOnly = 1, RST = 2 in
> +let hasSideEffects = 1, isCodeGenOnly = 1, RST = 2, Defs = [X2] in
>  def LDinto_toc: DSForm_1<58, 0, (outs), (ins memrix:$src),
>                      "ld 2, $src", IIC_LdStLD,
>                      [(PPCload_toc ixaddr:$src)]>, isPPC64;
> 
> 
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