[llvm] 45a0b81 - [InstCombine] Remove early constant fold

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2023-02-20T16:48:39+01:00
New Revision: 45a0b812fa13ec255cae91f974540a4d805a8d79

URL: https://github.com/llvm/llvm-project/commit/45a0b812fa13ec255cae91f974540a4d805a8d79
DIFF: https://github.com/llvm/llvm-project/commit/45a0b812fa13ec255cae91f974540a4d805a8d79.diff

LOG: [InstCombine] Remove early constant fold

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
call into simplification, and that's casts. 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 and in particular
mitigates the compile-time regression from enabling load
simplification in be88b5814d9efce131dbc0c8e288907e2e6c89be.

Differential Revision: https://reviews.llvm.org/D144369

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 71fe7fd558b0..d723b8b1abe7 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -308,6 +308,10 @@ Instruction *InstCombinerImpl::commonCastTransforms(CastInst &CI) {
   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)) {

diff  --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index d09a24608963..2679e0aec2b6 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -4221,23 +4221,6 @@ bool InstCombinerImpl::run() {
     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.

diff  --git a/llvm/test/Transforms/InstCombine/pr38677.ll b/llvm/test/Transforms/InstCombine/pr38677.ll
index bacee2c67676..f60c7fb209f2 100644
--- a/llvm/test/Transforms/InstCombine/pr38677.ll
+++ b/llvm/test/Transforms/InstCombine/pr38677.ll
@@ -11,9 +11,8 @@ define i32 @foo(i1 %which, ptr %dst) {
 ; 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