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LLVM backend for 6502
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------------------------------------------------------------------------ r215685 | wschmidt | 2014-08-14 18:25:26 -0700 (Thu, 14 Aug 2014) | 69 lines [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. Have we missed the boat for 3.5 at this point? Thanks, Bill ------------------------------------------------------------------------ git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_35@215878 91177308-0d34-0410-b5e6-96231b3b80d8 |
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