Re: [commit: ghc] simd: Fixup stack spills when generating AVX instructions. (b787b5d)

Do we have a longer term solution for this? I love the SIMD work, so please don't take this as a complaint, I agree with getting it done for now with the mangler and truth is it's doubtful we'll et around to adding TNTC support to LLVM anytime soon, so mangler is here to stay for now. BUT. I would like to see some thought about how in an ideal world with lots of free time to hack on GHC we'd kill the mangler. Do we know that plan for this patch? If so it may be nice to document it somewhere and perhaps create a trac ticket. On 14 February 2013 23:16, Geoffrey Mainland wrote:

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch : simd

http://hackage.haskell.org/trac/ghc/changeset/b787b5d1e687fb28643cdd6a847ccc...

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commit b787b5d1e687fb28643cdd6a847ccc26bb014a79 Author: Geoffrey Mainland Date: Sat Nov 26 12:45:23 2011 +0000

Fixup stack spills when generating AVX instructions.

LLVM uses aligned AVX moves to spill values onto the stack, which requires 32-bye aligned stacks. Since the stack in only 16-byte aligned, LLVM inserts extra instructions that munge the stack pointer. This is very very bad for the GHC calling convention, so we tell LLVM to assume the stack is 32-byte aligned. This patch rewrites the spill instructions that LLVM generates so they do not require an aligned stack.

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compiler/llvmGen/LlvmMangler.hs | 38 ++++++++++++++++++++++++++++++++++++++ 1 files changed, 38 insertions(+), 0 deletions(-)

diff --git a/compiler/llvmGen/LlvmMangler.hs b/compiler/llvmGen/LlvmMangler.hs index 83a2be7..745dcc6 100644 --- a/compiler/llvmGen/LlvmMangler.hs +++ b/compiler/llvmGen/LlvmMangler.hs @@ -20,6 +20,10 @@ import System.IO import Data.List ( sortBy ) import Data.Function ( on )

+#if x86_64_TARGET_ARCH +#define REWRITE_AVX +#endif + -- Magic Strings secStmt, infoSec, newLine, textStmt, dataStmt, syntaxUnified :: B.ByteString secStmt = B.pack "\t.section\t" @@ -47,6 +51,7 @@ llvmFixupAsm dflags f1 f2 = {-# SCC "llvm_mangler" #-} do w <- openBinaryFile f2 WriteMode ss <- readSections r w hClose r + let fixed = (map rewriteAVX . fixTables) ss let fixed = fixTables ss mapM_ (writeSection w) fixed hClose w @@ -90,6 +95,39 @@ writeSection w (hdr, cts) = do B.hPutStrLn w hdr B.hPutStrLn w cts

+#if REWRITE_AVX +rewriteAVX :: Section -> Section +rewriteAVX = rewriteVmovaps . rewriteVmovdqa + +rewriteVmovdqa :: Section -> Section +rewriteVmovdqa = rewriteInstructions vmovdqa vmovdqu + where + vmovdqa, vmovdqu :: B.ByteString + vmovdqa = B.pack "vmovdqa" + vmovdqu = B.pack "vmovdqu" + +rewriteVmovap :: Section -> Section +rewriteVmovap = rewriteInstructions vmovap vmovup + where + vmovap, vmovup :: B.ByteString + vmovap = B.pack "vmovap" + vmovup = B.pack "vmovup" + +rewriteInstructions :: B.ByteString -> B.ByteString -> Section -> Section +rewriteInstructions matchBS replaceBS (hdr, cts) = + (hdr, loop cts) + where + loop :: B.ByteString -> B.ByteString + loop cts = + case B.breakSubstring cts matchBS of + (hd,tl) | B.null tl -> hd + | otherwise -> hd `B.append` replaceBS `B.append` + loop (B.drop (B.length matchBS) tl) +#else /* !REWRITE_AVX */ +rewriteAVX :: Section -> Section +rewriteAVX = id +#endif /* !REWRITE_SSE */ + -- | Reorder and convert sections so info tables end up next to the -- code. Also does stack fixups. fixTables :: [Section] -> [Section]

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