[llvm] cc29600 - [SCEV] SCEVExpander::isHighCostExpansionHelper(): cost-model plain UDiv

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2020-02-25T23:05:58+03:00
New Revision: cc29600b908ba3aefb41e53398922319841fdb37

URL: https://github.com/llvm/llvm-project/commit/cc29600b908ba3aefb41e53398922319841fdb37
DIFF: https://github.com/llvm/llvm-project/commit/cc29600b908ba3aefb41e53398922319841fdb37.diff

LOG: [SCEV] SCEVExpander::isHighCostExpansionHelper(): cost-model plain UDiv

Summary:
If we don't believe this UDiv is actually a LShr in disguise, things are much worse.
First, we try to see if this UDiv actually originates from user code,
by looking for `S + 1`, and if found considering this UDiv to be free.
But otherwise, we always considered this UDiv to be high-cost.

However that is no longer the case with TTI-driven cost model:
our default budget is 4, which matches the default cost of UDiv,
so now we allow a single UDiv to not be counted as high-cost.

While that is the case, it is evident this is actually a regression
due to the fact that cost-modelling is incomplete - we did not account
for the `add`, `mul` costs yet. That is being addressed in D73728.

Cost-modelling for UDiv also seems pretty straight-forward:
subtract cost of the UDiv itself, and recurse into both the LHS and RHS.

Reviewers: reames, mkazantsev, wmi, sanjoy

Reviewed By: mkazantsev

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73722

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolutionExpander.cpp
    llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
index 73f70d4fe25f..abfb784f69f2 100644
--- a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -2196,20 +2196,27 @@ bool SCEVExpander::isHighCostExpansionHelper(
     // UDivExpr is very likely a UDiv that ScalarEvolution's HowFarToZero or
     // HowManyLessThans produced to compute a precise expression, rather than a
     // UDiv from the user's code. If we can't find a UDiv in the code with some
-    // simple searching, assume the former consider UDivExpr expensive to
-    // compute.
+    // simple searching, we need to account for it's cost.
+
     BasicBlock *ExitingBB = L->getExitingBlock();
-    if (!ExitingBB)
-      return true;
+    if (At || ExitingBB) {
+      if (!At)
+        At = &ExitingBB->back();
+
+      // At the beginning of this function we already tried to find existing
+      // value for plain 'S'. Now try to lookup 'S + 1' since it is common
+      // pattern involving division. This is just a simple search heuristic.
+      if (getRelatedExistingExpansion(
+              SE.getAddExpr(S, SE.getConstant(S->getType(), 1)), At, L))
+        return false; // Consider it to be free.
+    }
 
-    // At the beginning of this function we already tried to find existing value
-    // for plain 'S'. Now try to lookup 'S + 1' since it is common pattern
-    // involving division. This is just a simple search heuristic.
-    if (!At)
-      At = &ExitingBB->back();
-    if (!getRelatedExistingExpansion(
-            SE.getAddExpr(S, SE.getConstant(S->getType(), 1)), At, L))
-      return true;
+    // Need to count the cost of this UDiv.
+    BudgetRemaining -= TTI.getOperationCost(Instruction::UDiv, S->getType());
+    return isHighCostExpansionHelper(UDivExpr->getLHS(), L, At, BudgetRemaining,
+                                     TTI, Processed) ||
+           isHighCostExpansionHelper(UDivExpr->getRHS(), L, At, BudgetRemaining,
+                                     TTI, Processed);
   }
 
   // HowManyLessThans uses a Max expression whenever the loop is not guarded by

diff  --git a/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll b/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll
index 93cced597cac..08b3d62e518c 100644
--- a/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll
+++ b/llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll
@@ -19,6 +19,9 @@ define i32 @_Z3fooPKcjj(i8* nocapture readonly %s, i32 %len, i32 %c) {
 ; CHECK-NEXT:    [[CMP8:%.*]] = icmp ugt i32 [[LEN:%.*]], 11
 ; CHECK-NEXT:    br i1 [[CMP8]], label [[WHILE_BODY_LR_PH:%.*]], label [[WHILE_END:%.*]]
 ; CHECK:       while.body.lr.ph:
+; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[LEN]], -12
+; CHECK-NEXT:    [[TMP1:%.*]] = udiv i32 [[TMP0]], 12
+; CHECK-NEXT:    [[TMP2:%.*]] = mul i32 [[TMP1]], 12
 ; CHECK-NEXT:    br label [[WHILE_BODY:%.*]]
 ; CHECK:       while.body:
 ; CHECK-NEXT:    [[KEYLEN_010:%.*]] = phi i32 [ [[LEN]], [[WHILE_BODY_LR_PH]] ], [ [[SUB:%.*]], [[WHILE_BODY]] ]
@@ -36,10 +39,10 @@ define i32 @_Z3fooPKcjj(i8* nocapture readonly %s, i32 %len, i32 %c) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i32 [[SUB]], 11
 ; CHECK-NEXT:    br i1 [[CMP]], label [[WHILE_BODY]], label [[WHILE_COND_WHILE_END_CRIT_EDGE:%.*]]
 ; CHECK:       while.cond.while.end_crit_edge:
-; CHECK-NEXT:    [[SUB_LCSSA:%.*]] = phi i32 [ [[SUB]], [[WHILE_BODY]] ]
+; CHECK-NEXT:    [[TMP3:%.*]] = sub i32 [[TMP0]], [[TMP2]]
 ; CHECK-NEXT:    br label [[WHILE_END]]
 ; CHECK:       while.end:
-; CHECK-NEXT:    [[KEYLEN_0_LCSSA:%.*]] = phi i32 [ [[SUB_LCSSA]], [[WHILE_COND_WHILE_END_CRIT_EDGE]] ], [ [[LEN]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[KEYLEN_0_LCSSA:%.*]] = phi i32 [ [[TMP3]], [[WHILE_COND_WHILE_END_CRIT_EDGE]] ], [ [[LEN]], [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    call void @_Z3mixRjj(i32* dereferenceable(4) [[A]], i32 [[KEYLEN_0_LCSSA]])
 ; CHECK-NEXT:    [[T4:%.*]] = load i32, i32* [[A]], align 4
 ; CHECK-NEXT:    call void @llvm.lifetime.end.p0i8(i64 4, i8* [[T]])