[PATCH] Eliminate GC check when no extra stack is used.

[Edward Z. Yang]

Signed-off-by: Edward Z. Yang <ezyang at mit.edu>
---
 compiler/cmm/CmmLint.hs        |    1 +
 compiler/cmm/CmmStackLayout.hs |   13 +++++++++++++
 2 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/compiler/cmm/CmmLint.hs b/compiler/cmm/CmmLint.hs
index c14ad65..32fead3 100644
--- a/compiler/cmm/CmmLint.hs
+++ b/compiler/cmm/CmmLint.hs
@@ -153,6 +153,7 @@ lintTarget (CmmPrim {})    = return ()
 
 checkCond :: CmmExpr -> CmmLint ()
 checkCond (CmmMachOp mop _) | isComparisonMachOp mop = return ()
+checkCond (CmmLit (CmmInt x t)) | x == 0 || x == 1, t == wordWidth = return () -- constant values
 checkCond expr = cmmLintErr (hang (text "expression is not a conditional:") 2
 				    (ppr expr))
 
diff --git a/compiler/cmm/CmmStackLayout.hs b/compiler/cmm/CmmStackLayout.hs
index 64be51b..4be377e 100644
--- a/compiler/cmm/CmmStackLayout.hs
+++ b/compiler/cmm/CmmStackLayout.hs
@@ -518,6 +518,19 @@ manifestSP spEntryMap areaMap entry_off g@(CmmGraph {g_entry=entry}) =
                 replSlot spOff (CmmStackSlot a i) = CmmRegOff (CmmGlobal Sp) (spOff - (slot a + i))
                 replSlot _ (CmmLit CmmHighStackMark) = -- replacing the high water mark
                   CmmLit (CmmInt (toInteger (max 0 (sp_high - proc_entry_sp))) (typeWidth bWord))
+                -- Invariant: Sp is always less than SpLim.  Thus, if
+                -- the high water mark is zero, we can optimize away the
+                -- conditional branch.  Relies on dead code elimination
+                -- to get rid of the dead GC blocks.
+                -- EZY: Maybe turn this into a guard that checks if a
+                -- statement is stack-check ish?  Maybe we should make
+                -- an actual mach-op for it, so there's no chance of
+                -- mixing this up with something else...
+                replSlot _ (CmmMachOp (MO_U_Lt _)
+                              [CmmMachOp (MO_Sub _)
+                                         [ CmmReg (CmmGlobal Sp)
+                                         , CmmLit (CmmInt 0 _)],
+                               CmmReg (CmmGlobal SpLim)]) = CmmLit (CmmInt 0 wordWidth)
                 replSlot _ e = e
 
                 replLast :: MaybeC C (CmmNode C O) -> [CmmNode O O] -> CmmNode O C -> FuelUniqSM [CmmBlock]
-- 
1.7.4.3