[PATCH] D144369: [InstCombine] Remove early constant fold

[Nikita Popov via Phabricator via llvm-commits]

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InstCombine currently performs a constant folding attempt as part of the main InstCombine loop, before visiting the instruction. However, each visit method will also attempt to simplify the instruction, which will in turn constant fold it. (Additionally, we also constant fold instructions before the main InstCombine loop and use a constant folding IR builder, so this is doubly redundant.)

There is one place where InstCombine visit methods currently don't all into simplification, and that's cast. To be conservative, I've added an explicit constant folding call there (though it has no impact on tests).

This makes for a mild compile-time improvement (http://llvm-compile-time-tracker.com/compare.php?from=b15d70e57a440c37c11b0ad3173d7dc624e4fa07&to=1753e2859139af9a8a8551780c791e53ad20f90d&stat=instructions:u) and in particular mitigates the compile-time regression from enabling load simplification in https://github.com/llvm/llvm-project/commit/be88b5814d9efce131dbc0c8e288907e2e6c89be.


https://reviews.llvm.org/D144369

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/pr38677.ll


Index: llvm/test/Transforms/InstCombine/pr38677.ll
===================================================================
--- llvm/test/Transforms/InstCombine/pr38677.ll
+++ llvm/test/Transforms/InstCombine/pr38677.ll
@@ -11,9 +11,8 @@
 ; CHECK:       delay:
 ; CHECK-NEXT:    br label [[FINAL]]
 ; CHECK:       final:
-; CHECK-NEXT:    [[USE2:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ select (i1 icmp eq (ptr @A, ptr @B), i32 2, i32 1), [[DELAY]] ]
 ; CHECK-NEXT:    store i1 false, ptr [[DST:%.*]], align 1
-; CHECK-NEXT:    ret i32 [[USE2]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   br i1 true, label %final, label %delay
Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -4221,23 +4221,6 @@
     if (!DebugCounter::shouldExecute(VisitCounter))
       continue;
 
-    // Instruction isn't dead, see if we can constant propagate it.
-    if (!I->use_empty() &&
-        (I->getNumOperands() == 0 || isa<Constant>(I->getOperand(0)))) {
-      if (Constant *C = ConstantFoldInstruction(I, DL, &TLI)) {
-        LLVM_DEBUG(dbgs() << "IC: ConstFold to: " << *C << " from: " << *I
-                          << '\n');
-
-        // Add operands to the worklist.
-        replaceInstUsesWith(*I, C);
-        ++NumConstProp;
-        if (isInstructionTriviallyDead(I, &TLI))
-          eraseInstFromFunction(*I);
-        MadeIRChange = true;
-        continue;
-      }
-    }
-
     // See if we can trivially sink this instruction to its user if we can
     // prove that the successor is not executed more frequently than our block.
     // Return the UserBlock if successful.
Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -308,6 +308,10 @@
   Value *Src = CI.getOperand(0);
   Type *Ty = CI.getType();
 
+  if (auto *SrcC = dyn_cast<Constant>(Src))
+    if (Constant *Res = ConstantFoldCastOperand(CI.getOpcode(), SrcC, Ty, DL))
+      return replaceInstUsesWith(CI, Res);
+
   // Try to eliminate a cast of a cast.
   if (auto *CSrc = dyn_cast<CastInst>(Src)) {   // A->B->C cast
     if (Instruction::CastOps NewOpc = isEliminableCastPair(CSrc, &CI)) {


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