For x86, canonicalize max

(x > y) ? x : y
=>
(x >= y) ? x : y

So for something like
(x - y) > 0 : (x - y) ? 0
It will be
(x - y) >= 0 : (x - y) ? 0

This makes is possible to test sign-bit and eliminate a comparison against
zero. e.g.
subl   %esi, %edi
testl  %edi, %edi
movl   $0, %eax
cmovgl %edi, %eax
=>
xorl   %eax, %eax
subl   %esi, $edi
cmovsl %eax, %edi

rdar://10633221

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147512 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index e12343ebcf9416455d30ddb97ca4fcce8d38e07e..d33a7e09a56dcc910e3ec98696fceac249fd5a0e 100644 (file)
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -13139,6 +13139,37 @@ static SDValue PerformSELECTCombine(SDNode *N, SelectionDAG &DAG,
       }
   }
 
+  // Canonicalize max and min:
+  // (x > y) ? x : y -> (x >= y) ? x : y
+  // (x < y) ? x : y -> (x <= y) ? x : y
+  // This allows use of COND_S / COND_NS (see TranslateX86CC) which eliminates
+  // the need for an extra compare
+  // against zero. e.g.
+  // (x - y) > 0 : (x - y) ? 0 -> (x - y) >= 0 : (x - y) ? 0
+  // subl   %esi, %edi
+  // testl  %edi, %edi
+  // movl   $0, %eax
+  // cmovgl %edi, %eax
+  // =>
+  // xorl   %eax, %eax
+  // subl   %esi, $edi
+  // cmovsl %eax, %edi
+  if (N->getOpcode() == ISD::SELECT && Cond.getOpcode() == ISD::SETCC &&
+      DAG.isEqualTo(LHS, Cond.getOperand(0)) &&
+      DAG.isEqualTo(RHS, Cond.getOperand(1))) {
+    ISD::CondCode CC = cast<CondCodeSDNode>(Cond.getOperand(2))->get();
+    switch (CC) {
+    default: break;
+    case ISD::SETLT:
+    case ISD::SETGT: {
+      ISD::CondCode NewCC = (CC == ISD::SETLT) ? ISD::SETLE : ISD::SETGE;
+      Cond = DAG.getSetCC(Cond.getDebugLoc(), Cond.getValueType(),
+                          Cond.getOperand(0), Cond.getOperand(1), NewCC);
+      return DAG.getNode(ISD::SELECT, DL, VT, Cond, LHS, RHS);
+    }
+    }
+  }
+
   return SDValue();
 }
 

diff --git a/test/CodeGen/X86/jump_sign.ll b/test/CodeGen/X86/jump_sign.ll
index 5e8e16217363e82053f7b1f7a9e2e6e63a07db03..78dddf9568f13cfb0ff48cad2a3021dc842fcae7 100644 (file)
--- a/test/CodeGen/X86/jump_sign.ll
+++ b/test/CodeGen/X86/jump_sign.ll
@@ -1,7 +1,9 @@
-; RUN: llc < %s -march=x86 | grep jns
+; RUN: llc < %s -march=x86 | FileCheck %s
 
 define i32 @f(i32 %X) {
 entry:
+; CHECK: f:
+; CHECK: jns
        %tmp1 = add i32 %X, 1           ; <i32> [#uses=1]
        %tmp = icmp slt i32 %tmp1, 0            ; <i1> [#uses=1]
        br i1 %tmp, label %cond_true, label %cond_next
@@ -18,3 +20,15 @@ cond_next:           ; preds = %cond_true, %entry
 declare i32 @bar(...)
 
 declare i32 @baz(...)
+
+; rdar://10633221
+define i32 @g(i32 %a, i32 %b) nounwind {
+entry:
+; CHECK: g:
+; CHECK-NOT: test
+; CHECK: cmovs
+  %sub = sub nsw i32 %a, %b
+  %cmp = icmp sgt i32 %sub, 0
+  %cond = select i1 %cmp, i32 %sub, i32 0
+  ret i32 %cond
+}