[PATCH] D48447: Fix invariant fdiv hoisting in LICM

[Stanislav Mekhanoshin via Phabricator via llvm-commits]

rampitec created this revision.
rampitec added reviewers: hfinkel, reames.

FDiv is replaced with multiplication by reciprocal and invariant
reciprocal is hoisted out of the loop, while multiplication remains
even if invariant.

Switch checks for all invariant operands and only invariant
denominator to fix the issue.


https://reviews.llvm.org/D48447

Files:
  lib/Transforms/Scalar/LICM.cpp
  test/Transforms/LICM/hoist-fast-fdiv.ll


Index: test/Transforms/LICM/hoist-fast-fdiv.ll
===================================================================
--- test/Transforms/LICM/hoist-fast-fdiv.ll
+++ test/Transforms/LICM/hoist-fast-fdiv.ll
@@ -1,9 +1,9 @@
 ; RUN: opt -licm -S < %s | FileCheck %s
 
 ; Function Attrs: noinline norecurse nounwind readnone ssp uwtable
-define zeroext i1 @f(double %v) #0 {
+define zeroext i1 @invariant_denom(double %v) #0 {
 entry:
-; CHECK-LABEL: @f(
+; CHECK-LABEL: @invariant_denom(
 ; CHECK-NEXT: entry:
 ; CHECK-NEXT: fdiv fast double 1.000000e+00, %v
   br label %loop
@@ -32,3 +32,29 @@
   ret i1 %v16
 }
 
+define void @invariant_fdiv(float* %out, float %arg) {
+; CHECK-LABEL: @invariant_fdiv(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: %div = fdiv fast float 4.000000e+00, %arg
+; CHECK-NEXT: fmul fast float %div, 0x41F0000000000000
+entry:
+  br label %loop
+
+loop:                                              ; preds = %loop, %entry
+  %ind = phi i32 [ 0, %entry ], [ %inc, %loop ]
+
+; CHECK-LABEL: loop:
+; CHECK: getelementptr
+; CHECK-NOT: fdiv
+; CHECK-NOT: fmul
+  %div = fdiv fast float 4.000000e+00, %arg
+  %mul = fmul fast float %div, 0x41F0000000000000
+  %gep = getelementptr inbounds float, float* %out, i32 %ind
+  store float %mul, float* %gep, align 4
+  %inc = add nuw nsw i32 %ind, 1
+  %cond = icmp eq i32 %inc, 1024
+  br i1 %cond, label %exit, label %loop
+
+exit:                                              ; preds = %loop
+  ret void
+}
Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -477,6 +477,20 @@
         continue;
       }
 
+      // Try hoisting the instruction out to the preheader.  We can only do
+      // this if all of the operands of the instruction are loop invariant and
+      // if it is safe to hoist the instruction.
+      //
+      if (CurLoop->hasLoopInvariantOperands(&I) &&
+          canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE) &&
+          (IsMustExecute ||
+           isSafeToExecuteUnconditionally(
+               I, DT, CurLoop, SafetyInfo, ORE,
+               CurLoop->getLoopPreheader()->getTerminator()))) {
+        Changed |= hoist(I, DT, CurLoop, SafetyInfo, ORE);
+        continue;
+      }
+
       // Attempt to remove floating point division out of the loop by
       // converting it to a reciprocal multiplication.
       if (I.getOpcode() == Instruction::FDiv &&
@@ -500,20 +514,6 @@
         continue;
       }
 
-      // Try hoisting the instruction out to the preheader.  We can only do
-      // this if all of the operands of the instruction are loop invariant and
-      // if it is safe to hoist the instruction.
-      //
-      if (CurLoop->hasLoopInvariantOperands(&I) &&
-          canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE) &&
-          (IsMustExecute ||
-           isSafeToExecuteUnconditionally(
-               I, DT, CurLoop, SafetyInfo, ORE,
-               CurLoop->getLoopPreheader()->getTerminator()))) {
-        Changed |= hoist(I, DT, CurLoop, SafetyInfo, ORE);
-        continue;
-      }
-
       if (IsMustExecute)
         IsMustExecute = isGuaranteedToTransferExecutionToSuccessor(&I);
     }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D48447.152348.patch
Type: text/x-patch
Size: 3277 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180621/b11e1b0e/attachment.bin>