[CodeGenPrepare] Avoid sinking soft-FP comparisons

whitequark · whitequark · commit 0b6d7bc68207 · 2016-04-03T16:36:17.000Z
Sinking comparisons in CGP can undo the job of hoisting them done earlier by LICM, and soft-FP makes this an expensive mistake. A common pattern that produces floating point comparisons uniform over a loop is an explicit check for division by zero. If the divisor is hoisted out of the loop, the comparison can also be, but hoisting the function that unwinds is never legal, since it may cause side effects in the loop body prior to the unwinding to not be executed. Differential Revision: http://reviews.llvm.org/D18744 llvm-svn: 265264
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -855,10 +855,14 @@ static bool CombineUAddWithOverflow(CmpInst *CI) {
 /// lose; some adjustment may be wanted there.
 ///
 /// Return true if any changes are made.
-static bool SinkCmpExpression(CmpInst *CI) {
+static bool SinkCmpExpression(CmpInst *CI, const TargetLowering &TLI) {
   BasicBlock *DefBB = CI->getParent();
 
-  /// Only insert a cmp in each block once.
+  // Avoid sinking soft-FP comparisons, since this can move them into a loop.
+  if (TLI.useSoftFloat() && isa<FCmpInst>(CI))
+    return false;
+
+  // Only insert a cmp in each block once.
   DenseMap<BasicBlock*, CmpInst*> InsertedCmps;
 
   bool MadeChange = false;
@@ -906,8 +910,8 @@ static bool SinkCmpExpression(CmpInst *CI) {
   return MadeChange;
 }
 
-static bool OptimizeCmpExpression(CmpInst *CI) {
-  if (SinkCmpExpression(CI))
+static bool OptimizeCmpExpression(CmpInst *CI, const TargetLowering &TLI) {
+  if (SinkCmpExpression(CI, TLI))
     return true;
 
   if (CombineUAddWithOverflow(CI))
@@ -5173,7 +5177,7 @@ bool CodeGenPrepare::optimizeInst(Instruction *I, bool& ModifiedDT) {
 
   if (CmpInst *CI = dyn_cast<CmpInst>(I))
     if (!TLI || !TLI->hasMultipleConditionRegisters())
-      return OptimizeCmpExpression(CI);
+      return OptimizeCmpExpression(CI, *TLI);
 
   if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
     stripInvariantGroupMetadata(*LI);
diff --git a/llvm/test/Transforms/CodeGenPrepare/X86/fcmp-sinking.ll b/llvm/test/Transforms/CodeGenPrepare/X86/fcmp-sinking.ll
@@ -0,0 +1,29 @@
+; RUN: opt %s -codegenprepare -mattr=+soft-float -S | FileCheck %s -check-prefix=CHECK -check-prefix=SOFTFP
+; RUN: opt %s -codegenprepare -mattr=-soft-float -S | FileCheck %s -check-prefix=CHECK -check-prefix=HARDFP
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @foo
+; CHECK:       entry:
+; SOFTFP:      fcmp
+; HARDFP-NOT:  fcmp
+; CHECK:       body:
+; SOFTFP-NOT:  fcmp
+; HARDFP:      fcmp
+define void @foo(float %a, float %b) {
+entry:
+  %c = fcmp oeq float %a, %b
+  br label %head
+head:
+  %IND = phi i32 [ 0, %entry ], [ %IND.new, %body1 ]
+  %CMP = icmp slt i32 %IND, 1250
+  br i1 %CMP, label %body, label %tail
+body:
+  br i1 %c, label %body1, label %tail
+body1:
+  %IND.new = add i32 %IND, 1
+  br label %head
+tail:
+  ret void
+}
