Generate (old + 0) instead of Sp in stack checks

When compiling a function we can determine how much stack space it will
use. We therefore need to perform only a single stack check at the beginning
of a function to see if we have enough stack space. Instead of referring
directly to Sp - as we used to do in the past - the code generator uses
(old + 0) in the stack check. Stack layout phase turns (old + 0) into Sp.

The idea here is that, while we need to perform only one stack check for
each function, we could in theory place more stack checks later in the
function. They would be redundant, but not incorrect (in a sense that they
should not change program behaviour). We need to make sure however that a
stack check inserted after incrementing the stack pointer checks for a
respectively smaller stack space. This would not be the case if the code
generator produced direct references to Sp. By referencing (old + 0) we make
sure that we always check for a correct amount of stack: when converting
(old + 0) to Sp the stack layout phase takes into account changes already
made to stack pointer. The idea for this change came from observations made
while debugging #8275.

1 files changed, 25 insertions, 2 deletions

diff --git a/compiler/codeGen/StgCmmHeap.hs b/compiler/codeGen/StgCmmHeap.hs
index f4c58e95e1..1d1100c6be 100644
--- a/compiler/codeGen/StgCmmHeap.hs
+++ b/compiler/codeGen/StgCmmHeap.hs
@@ -533,6 +533,27 @@ heapStackCheckGen stk_hwm mb_bytes
        call <- mkCall generic_gc (GC, GC) [] [] updfr_sz []
        do_checks stk_hwm False  mb_bytes (call <*> mkBranch lretry)
 
+-- Note [Single stack check]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~
+--
+-- When compiling a function we can determine how much stack space it will
+-- use. We therefore need to perform only a single stack check at the beginning
+-- of a function to see if we have enough stack space. Instead of referring
+-- directly to Sp - as we used to do in the past - the code generator uses
+-- (old + 0) in the stack check. Stack layout phase turns (old + 0) into Sp.
+--
+-- The idea here is that, while we need to perform only one stack check for
+-- each function, we could in theory place more stack checks later in the
+-- function. They would be redundant, but not incorrect (in a sense that they
+-- should not change program behaviour). We need to make sure however that a
+-- stack check inserted after incrementing the stack pointer checks for a
+-- respectively smaller stack space. This would not be the case if the code
+-- generator produced direct references to Sp. By referencing (old + 0) we make
+-- sure that we always check for a correct amount of stack: when converting
+-- (old + 0) to Sp the stack layout phase takes into account changes already
+-- made to stack pointer. The idea for this change came from observations made
+-- while debugging #8275.
+
 do_checks :: Maybe CmmExpr    -- Should we check the stack?
           -> Bool       -- Should we check for preemption?
           -> Maybe CmmExpr    -- Heap headroom (bytes)
@@ -547,11 +568,13 @@ do_checks mb_stk_hwm checkYield mb_alloc_lit do_gc = do
 
     bump_hp   = cmmOffsetExprB dflags (CmmReg hpReg) alloc_lit
 
-    -- Sp overflow if (Sp - CmmHighStack < SpLim)
+    -- Sp overflow if ((old + 0) - CmmHighStack < SpLim)
+    -- At the beginning of a function old + 0 = Sp
+    -- See Note [Single stack check]
     sp_oflo sp_hwm =
          CmmMachOp (mo_wordULt dflags)
                   [CmmMachOp (MO_Sub (typeWidth (cmmRegType dflags spReg)))
-                             [CmmReg spReg, sp_hwm],
+                             [CmmStackSlot Old 0, sp_hwm],
                    CmmReg spLimReg]
 
     -- Hp overflow if (Hp > HpLim)