Allow aligning of cmm procs at specific boundry

Allows to align CmmProcs at the given boundries.

It makes performance usually worse but can be helpful
to limit the effect of a unrelated function B becoming
faster/slower after changing function A.

Test Plan: ci, using it.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15148

Differential Revision: https://phabricator.haskell.org/D4706

3 files changed, 25 insertions, 0 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index b9141f9e87..b2c82fa2ee 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -876,6 +876,8 @@ data DynFlags = DynFlags {
   floatLamArgs          :: Maybe Int,   -- ^ Arg count for lambda floating
                                         --   See CoreMonad.FloatOutSwitches
 
+  cmmProcAlignment      :: Maybe Int,   -- ^ Align Cmm functions at this boundry or use default.
+
   historySize           :: Int,         -- ^ Simplification history size
 
   importPaths           :: [FilePath],
@@ -1758,6 +1760,7 @@ defaultDynFlags mySettings (myLlvmTargets, myLlvmPasses) =
         specConstrRecursive     = 3,
         liberateCaseThreshold   = Just 2000,
         floatLamArgs            = Just 0, -- Default: float only if no fvs
+        cmmProcAlignment        = Nothing,
 
         historySize             = 20,
         strictnessBefore        = [],
@@ -3397,6 +3400,10 @@ dynamic_flags_deps = [
       (intSuffix (\n d -> d { floatLamArgs = Just n }))
   , make_ord_flag defFlag "ffloat-all-lams"
       (noArg (\d -> d { floatLamArgs = Nothing }))
+  , make_ord_flag defFlag "fproc-alignment"
+      (intSuffix (\n d -> d { cmmProcAlignment = Just n }))
+
+
   , make_ord_flag defFlag "fhistory-size"
       (intSuffix (\n d -> d { historySize = n }))
   , make_ord_flag defFlag "funfolding-creation-threshold"
diff --git a/compiler/nativeGen/X86/Ppr.hs b/compiler/nativeGen/X86/Ppr.hs
index c03bf4f14c..c5fbeb544e 100644
--- a/compiler/nativeGen/X86/Ppr.hs
+++ b/compiler/nativeGen/X86/Ppr.hs
@@ -73,12 +73,17 @@ import Data.Bits
 -- .subsections_via_symbols and -dead_strip can be found at
 -- <https://developer.apple.com/library/mac/documentation/DeveloperTools/Reference/Assembler/040-Assembler_Directives/asm_directives.html#//apple_ref/doc/uid/TP30000823-TPXREF101>
 
+pprProcAlignment :: SDoc
+pprProcAlignment = sdocWithDynFlags $ \dflags ->
+  (maybe empty pprAlign . cmmProcAlignment $ dflags)
+
 pprNatCmmDecl :: NatCmmDecl (Alignment, CmmStatics) Instr -> SDoc
 pprNatCmmDecl (CmmData section dats) =
   pprSectionAlign section $$ pprDatas dats
 
 pprNatCmmDecl proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
   sdocWithDynFlags $ \dflags ->
+  pprProcAlignment $$
   case topInfoTable proc of
     Nothing ->
        case blocks of
@@ -86,6 +91,7 @@ pprNatCmmDecl proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
            pprLabel lbl
          blocks -> -- special case for code without info table:
            pprSectionAlign (Section Text lbl) $$
+           pprProcAlignment $$
            pprLabel lbl $$ -- blocks guaranteed not null, so label needed
            vcat (map (pprBasicBlock top_info) blocks) $$
            (if debugLevel dflags > 0
@@ -95,6 +101,7 @@ pprNatCmmDecl proc@(CmmProc top_info lbl _ (ListGraph blocks)) =
     Just (Statics info_lbl _) ->
       sdocWithPlatform $ \platform ->
       pprSectionAlign (Section Text info_lbl) $$
+      pprProcAlignment $$
       (if platformHasSubsectionsViaSymbols platform
           then ppr (mkDeadStripPreventer info_lbl) <> char ':'
           else empty) $$
diff --git a/docs/users_guide/debugging.rst b/docs/users_guide/debugging.rst
index 4e0be937f4..7adcc84f13 100644
--- a/docs/users_guide/debugging.rst
+++ b/docs/users_guide/debugging.rst
@@ -758,6 +758,17 @@ Checking for consistency
     Compile with alignment checks for all info table dereferences. This can be
     useful when finding pointer tagging issues.
 
+.. ghc-flag:: -fproc-alignment
+    :shortdesc: Align functions at given boundry.
+    :type: dynamic
+
+    Align functions to multiples of the given value. Only valid values are powers
+    of two.
+
+    ``-fproc-alignment=64`` can be used to limit alignment impact on performance
+    as each function will start at a cache line.
+    However forcing larger alignments in general reduces performance.
+
 .. ghc-flag:: -fcatch-bottoms
     :shortdesc: Insert ``error`` expressions after bottoming expressions; useful
         when debugging the compiler.