[PATCH] D125774: [IndVars] Fold int->fp->int cast for small PHI node

[Allen zhong via Phabricator via llvm-commits]

Allen updated this revision to Diff 438116.
Allen retitled this revision from "[InstCombine] fold float trunc into exact itofp for small constants" to "[IndVars] Fold int->fp->int cast for small PHI node".
Allen edited the summary of this revision.
Allen added a comment.

redo base in SCEV according comment


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D125774/new/

https://reviews.llvm.org/D125774

Files:
  llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
  llvm/test/Transforms/IndVarSimplify/floating-point-small-iv.ll


Index: llvm/test/Transforms/IndVarSimplify/floating-point-small-iv.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/IndVarSimplify/floating-point-small-iv.ll
@@ -0,0 +1,37 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -indvars -adce -S | FileCheck %s
+
+ at array = dso_local global [101 x i32] zeroinitializer, align 4
+
+define void @small_const_bound(i32 %index) {
+; CHECK-LABEL: @small_const_bound(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[I_010_INT:%.*]] = phi i32 [ 100, [[ENTRY:%.*]] ], [ [[DEC_INT:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_010_INT]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds [101 x i32], [101 x i32]* @array, i64 0, i64 [[IDXPROM]]
+; CHECK-NEXT:    store i32 [[I_010_INT]], i32* [[ARRAYIDX]], align 4
+; CHECK-NEXT:    [[DEC_INT]] = add nsw i32 [[I_010_INT]], -1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i32 [[DEC_INT]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY]], label [[CLEANUP:%.*]]
+; CHECK:       cleanup:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %i.010 = phi float [ 1.000000e+02, %entry ], [ %dec, %for.body ]
+  %conv = fptosi float %i.010 to i32
+  %idxprom = sext i32 %conv to i64
+  %arrayidx = getelementptr inbounds [101 x i32], [101 x i32]* @array, i64 0, i64 %idxprom
+  store i32 %conv, i32* %arrayidx, align 4
+  %dec = fadd fast float %i.010, -1.000000e+00
+  %cmp = fcmp fast ogt float %dec, 0.000000e+00
+  br i1 %cmp, label %for.body, label %cleanup
+
+cleanup:                                 ; preds = %for.body
+  ret void
+}
+
Index: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -392,9 +392,31 @@
   //
   // We give preference to sitofp over uitofp because it is faster on most
   // platforms.
+  assert(isInt<32>(InitValue) && isInt<32>(IncValue) && isInt<32>(ExitValue) &&
+         "Integer value of a non-constant number");
   if (WeakPH) {
     Value *Conv = new SIToFPInst(NewPHI, PN->getType(), "indvar.conv",
                                  &*PN->getParent()->getFirstInsertionPt());
+
+    // Now we only support NewPHI type i32, so if [abs(IncValue),
+    // abs(ExitValue)] is in the range of (1 << DestNumSigBits), then we can use
+    // NewPHI directly.
+    // TODO: make use of these low zero bits of values IncValue and ExitValue.
+    int DestNumSigBits = PN->getType()->getFPMantissaWidth();
+    if ((std::abs(InitValue) <= (1 << DestNumSigBits)) &&
+        (std::abs(ExitValue) <= (1 << DestNumSigBits))) {
+      for (User *U : PN->users()) {
+        if (auto *CI = dyn_cast<CastInst>(U)) {
+          CastInst::CastOps Opcode = CI->getOpcode();
+          assert((Opcode == CastInst::FPToSI || Opcode == CastInst::FPToUI) &&
+                 "Unexpected cast");
+          CI->replaceUsesWithIf(
+              NewPHI, [&](Use &U) -> bool { return DT->dominates(NewPHI, U); });
+        }
+      }
+    }
+
+    // Use Conv to replace rest items that do not meet the above requirements.
     PN->replaceAllUsesWith(Conv);
     RecursivelyDeleteTriviallyDeadInstructions(PN, TLI, MSSAU.get());
   }


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