compiler: fix data section alignment in the wasm NCG

Previously we tried to lower the alignment requirement as far as
possible, based on the section kind inferred from the CLabel. For info
tables, .p2align 1 was applied given the GC should only need the
lowest bit to tag forwarding pointers. But this would lead to
unaligned loads/stores, which has a performance penalty even if the
wasm spec permits it. Furthermore, the test suite has shown memory
corruption in a few cases when compacting gc is used.

This patch takes a more conservative approach: all data sections
except C strings align to word size.

1 files changed, 10 insertions, 10 deletions

diff --git a/compiler/GHC/CmmToAsm/Wasm/FromCmm.hs b/compiler/GHC/CmmToAsm/Wasm/FromCmm.hs
index 52cba6f0a8..8e4f285d27 100644
--- a/compiler/GHC/CmmToAsm/Wasm/FromCmm.hs
+++ b/compiler/GHC/CmmToAsm/Wasm/FromCmm.hs
@@ -123,15 +123,15 @@ alignmentFromWordType TagI32 = mkAlignment 4
 alignmentFromWordType TagI64 = mkAlignment 8
 alignmentFromWordType _ = panic "alignmentFromWordType: unreachable"
 
--- | Calculate a data section's alignment. Closures needs to be
--- naturally aligned; info tables need to align to 2, so to get 1 tag
--- bit as forwarding pointer marker. The rest have no alignment
--- requirements.
-alignmentFromCmmSection :: WasmTypeTag w -> CLabel -> Alignment
-alignmentFromCmmSection t lbl
-  | isStaticClosureLabel lbl = alignmentFromWordType t
-  | isInfoTableLabel lbl = mkAlignment 2
-  | otherwise = mkAlignment 1
+-- | Calculate a data section's alignment. As a conservative
+-- optimization, a data section with a single CmmString/CmmFileEmbed
+-- has no alignment requirement, otherwise we always align to the word
+-- size to satisfy pointer tagging requirements and avoid unaligned
+-- loads/stores.
+alignmentFromCmmSection :: WasmTypeTag w -> [DataSectionContent] -> Alignment
+alignmentFromCmmSection _ [DataASCII {}] = mkAlignment 1
+alignmentFromCmmSection _ [DataIncBin {}] = mkAlignment 1
+alignmentFromCmmSection t _ = alignmentFromWordType t
 
 -- | Lower a 'CmmStatic'.
 lower_CmmStatic :: CmmStatic -> WasmCodeGenM w DataSectionContent
@@ -1650,7 +1650,7 @@ onCmmData lbl s statics = do
           { dataSectionKind =
               dataSectionKindFromCmmSection s,
             dataSectionAlignment =
-              alignmentFromCmmSection ty_word lbl,
+              alignmentFromCmmSection ty_word cs,
             dataSectionContents =
               case cs of
                 [DataASCII buf] -> [DataASCII $ buf `BS.snoc` 0]