From ea44eadfb9d269d06b889fbfe41286bf0c7a730d Mon Sep 17 00:00:00 2001
From: Johan Tibell <johan.tibell@gmail.com>
Date: Fri, 6 May 2011 14:13:03 +0200
Subject: [PATCH] Implement forward substitution of constants in the Cmm
 mini-inliner

Currently the mini-inliner would only forward substitute assignments
to registers that were used exactly once, to not risk duplicating
computation.  For constants there's no such risk so we always
substitute.  Prior to the change the Cmm

    fn
    {
        bits64 a, b;

        a = 1;
        b = a + a;
        RET_N(b);
    }

would be optimized as

    fn()    { []
            }
        ca: _cb::I64 = 1;
            R1 = _cb::I64 + _cb::I64;
            jump (I64[Sp + 0]) ();
    }

but after it would be optimized as

    fn()    { []
            }
        ca: R1 = 2;
            jump (I64[Sp + 0]) ();
    }

Note that this pass does not deal with the now dead assignment.
---
 compiler/cmm/CmmOpt.hs |   52 ++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/compiler/cmm/CmmOpt.hs b/compiler/cmm/CmmOpt.hs
index a2eecd5..1355cd2 100644
--- a/compiler/cmm/CmmOpt.hs
+++ b/compiler/cmm/CmmOpt.hs
@@ -99,11 +99,13 @@ cmmEliminateDeadBlocks blocks@(BasicBlock base_id _:_) =
 -- The mini-inliner
 
 {-
-This pass inlines assignments to temporaries that are used just
-once.  It works as follows:
+This pass inlines assignments to temporaries.  Temporaries that are
+only used once are unconditionally inlined.  Temporaries that are used
+two or more times are only inlined if they are assigned a literal.  It
+works as follows:
 
   - count uses of each temporary
-  - for each temporary that occurs just once:
+  - for each temporary:
 	- attempt to push it forward to the statement that uses it
         - only push forward past assignments to other temporaries
 	  (assumes that temporaries are single-assignment)
@@ -158,7 +160,24 @@ cmmMiniInline blocks = map do_inline blocks
 
 cmmMiniInlineStmts :: UniqFM Int -> [CmmStmt] -> [CmmStmt]
 cmmMiniInlineStmts uses [] = []
-cmmMiniInlineStmts uses (stmt@(CmmAssign (CmmLocal (LocalReg u _)) expr) : stmts)
+cmmMiniInlineStmts uses (stmt@(CmmAssign (CmmLocal (LocalReg u _)) expr@(CmmLit _)) : stmts)
+        -- not used: just discard this assignment
+  | Nothing <- lookupUFM uses u
+  = cmmMiniInlineStmts uses stmts
+
+        -- used: try to inline at all the use sites
+  | Just n <- lookupUFM uses u
+  =
+#ifdef NCG_DEBUG
+     trace ("nativeGen: inlining " ++ showSDoc (pprStmt stmt)) $
+#endif
+     case lookForInlineLit u expr stmts of
+         (m, stmts')
+             | n == m -> cmmMiniInlineStmts (delFromUFM uses u) stmts'
+             | otherwise ->
+                 stmt : cmmMiniInlineStmts (adjustUFM (\x -> x - m) uses u) stmts'
+
+cmmMiniInlineStmts uses (stmt@(CmmAssign (CmmLocal (LocalReg u _)) expr : stmts))
         -- not used at all: just discard this assignment
   | Nothing <- lookupUFM uses u
   = cmmMiniInlineStmts uses stmts
@@ -175,6 +194,31 @@ cmmMiniInlineStmts uses (stmt@(CmmAssign (CmmLocal (LocalReg u _)) expr) : stmts
 cmmMiniInlineStmts uses (stmt:stmts)
   = stmt : cmmMiniInlineStmts uses stmts
 
+-- | Takes a register, a 'CmmLit' expression assigned to that
+-- register, and a list of statements.  Inlines the expression at all
+-- use sites of the register.  Returns the number of substituations
+-- made and the, possibly modified, list of statements.
+lookForInlineLit :: Unique -> CmmExpr -> [CmmStmt] -> (Int, [CmmStmt])
+lookForInlineLit _ _ [] = (0, [])
+lookForInlineLit u expr stmts@(stmt : rest)
+  | Just n <- lookupUFM (countUses stmt) u
+  = case lookForInlineLit u expr rest of
+      (m, stmts) -> let z = n + m
+                    in z `seq` (z, inlineStmt u expr stmt : stmts)
+
+  | ok_to_skip
+  = case lookForInlineLit u expr rest of
+      (n, stmts) -> (n, stmt : stmts)
+
+  | otherwise
+  = (0, stmts)
+  where
+    -- We skip over assignments to registers, unless the register
+    -- being assigned to is the one we're inlining.
+    ok_to_skip = case stmt of
+        CmmAssign (CmmLocal r@(LocalReg u' _)) _ | u' == u -> False
+        _other -> True
+
 lookForInline u expr stmts = lookForInline' u expr regset stmts
     where regset = foldRegsUsed extendRegSet emptyRegSet expr
 
-- 
1.7.10.4