Add `Opt_CoreConstantFolding` to turn on constant folding (#20500)

Previously, `-O1` and `-O2`, by way of their effect on the compilation
pipeline, they implicitly turned on constant folding

5 files changed, 19 insertions, 3 deletions

diff --git a/compiler/GHC/Core/Opt/Pipeline.hs b/compiler/GHC/Core/Opt/Pipeline.hs
index 770d68ecc9..7062865ed7 100644
--- a/compiler/GHC/Core/Opt/Pipeline.hs
+++ b/compiler/GHC/Core/Opt/Pipeline.hs
@@ -130,6 +130,7 @@ getCoreToDo logger dflags
     phases        = simplPhases        dflags
     max_iter      = maxSimplIterations dflags
     rule_check    = ruleCheck          dflags
+    const_fold    = gopt Opt_CoreConstantFolding          dflags
     call_arity    = gopt Opt_CallArity                    dflags
     exitification = gopt Opt_Exitification                dflags
     strictness    = gopt Opt_Strictness                   dflags
@@ -150,7 +151,7 @@ getCoreToDo logger dflags
     profiling     = ways dflags `hasWay` WayProf
 
     do_presimplify = do_specialise -- TODO: any other optimizations benefit from pre-simplification?
-    do_simpl3      = rules_on      -- TODO: any other optimizations benefit from three-phase simplification?
+    do_simpl3      = const_fold || rules_on -- TODO: any other optimizations benefit from three-phase simplification?
 
     maybe_rule_check phase = runMaybe rule_check (CoreDoRuleCheck phase)
 
diff --git a/compiler/GHC/Driver/Config/CmmToAsm.hs b/compiler/GHC/Driver/Config/CmmToAsm.hs
index 3e075ee049..edf82a37cc 100644
--- a/compiler/GHC/Driver/Config/CmmToAsm.hs
+++ b/compiler/GHC/Driver/Config/CmmToAsm.hs
@@ -32,9 +32,9 @@ initNCGConfig dflags this_mod = NCGConfig
    , ncgCfgBlockLayout        = gopt Opt_CfgBlocklayout dflags
    , ncgCfgWeightlessLayout   = gopt Opt_WeightlessBlocklayout dflags
 
-     -- With -O1 and greater, the cmmSink pass does constant-folding, so
+     -- When constant-folding is enabled, the cmmSink pass does constant-folding, so
      -- we don't need to do it again in the native code generator.
-   , ncgDoConstantFolding     = optLevel dflags < 1
+   , ncgDoConstantFolding     = not (gopt Opt_CoreConstantFolding dflags || gopt Opt_CmmSink dflags)
 
    , ncgDumpRegAllocStages    = dopt Opt_D_dump_asm_regalloc_stages dflags
    , ncgDumpAsmStats          = dopt Opt_D_dump_asm_stats dflags
diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index e7205a8620..c66cb85bfe 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -228,6 +228,7 @@ data GeneralFlag
    | Opt_AlignmentSanitisation
    | Opt_CatchBottoms
    | Opt_NumConstantFolding
+   | Opt_CoreConstantFolding
    | Opt_FastPAPCalls                  -- #6084
 
    -- PreInlining is on by default. The option is there just to see how
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index f7d24d1fd5..def183b62b 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3378,6 +3378,7 @@ fFlagsDeps = [
   flagSpec "catch-bottoms"                    Opt_CatchBottoms,
   flagSpec "alignment-sanitisation"           Opt_AlignmentSanitisation,
   flagSpec "num-constant-folding"             Opt_NumConstantFolding,
+  flagSpec "core-constant-folding"            Opt_CoreConstantFolding,
   flagSpec "fast-pap-calls"                   Opt_FastPAPCalls,
   flagSpec "cmm-control-flow"                 Opt_CmmControlFlow,
   flagSpec "show-warning-groups"              Opt_ShowWarnGroups,
@@ -3847,6 +3848,8 @@ optLevelFlags -- see Note [Documenting optimisation flags]
     , ([0],     Opt_IgnoreInterfacePragmas)
     , ([0],     Opt_OmitInterfacePragmas)
 
+    , ([1,2],   Opt_CoreConstantFolding)
+
     , ([1,2],   Opt_CallArity)
     , ([1,2],   Opt_Exitification)
     , ([1,2],   Opt_CaseMerge)
diff --git a/docs/users_guide/using-optimisation.rst b/docs/users_guide/using-optimisation.rst
index 5fa9a81a5e..36fc057890 100644
--- a/docs/users_guide/using-optimisation.rst
+++ b/docs/users_guide/using-optimisation.rst
@@ -121,6 +121,17 @@ on by default are enabled by ``-O``, and as such you shouldn't
 need to set any of them explicitly. A flag ``-fwombat`` can be negated
 by saying ``-fno-wombat``.
 
+.. ghc-flag:: -fcore-constant-folding
+    :shortdesc: Enable constant folding in Core. Implied by :ghc-flag:`-O`.
+    :type: dynamic
+    :reverse: -fno-core-constant-folding
+    :category:
+
+    :default: on
+
+    Enables Core-level constant folding, i.e. propagation of values
+    that can be computed at compile time.
+
 .. ghc-flag:: -fcase-merge
     :shortdesc: Enable case-merging. Implied by :ghc-flag:`-O`.
     :type: dynamic