use "unpredictable" metadata in SelectionDAG when splitting compares

[spatel]

Summary

This patch uses the metadata created by the llvm.unpredictable intrinsic in D12341 to avoid creating an unpredictable branch.

[arsenm]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

1601–1602 ↗

(On Diff #33145)

bikeshed: Check the isJumpExpensive first

[arsenm]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

1601–1602 ↗

(On Diff #33145)

and after the hasOneUse. The metadata is the weirdest case reason to not this, so it makes sense to be the last thing checked

[spatel]

Patch updated:

1. Reordered branch conditions
2. Added a temp variable for the opcode, so we don't repeat getOpcode() three times

[spatel]

Thanks, Matt - reordered predicates.

[spatel]

Patch updated:
Added 'TODO' comment assuming that 'unpredictable' now carries a value rather being a boolean.

[spatel]

Patch updated:
Removed comment about threshold value; we can add that feature when we have profile data to feed into this metadata.

llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

void SelectionDAGBuilder::visitBr(const BranchInst &I) {
   //     jnz foo
   // Emit:
   //     cmp A, B
   //     je foo
   //     cmp D, E
   //     jle foo
   //
   if (const BinaryOperator *BOp = dyn_cast<BinaryOperator>(CondVal)) {
-    if (!DAG.getTargetLoweringInfo().isJumpExpensive() &&
-        BOp->hasOneUse() && (BOp->getOpcode() == Instruction::And ||
-                             BOp->getOpcode() == Instruction::Or)) {
+    Instruction::BinaryOps Opcode = BOp->getOpcode();
+    if (!DAG.getTargetLoweringInfo().isJumpExpensive() && BOp->hasOneUse() &&
+        !I.getMetadata(LLVMContext::MD_unpredictable) &&
+        (Opcode == Instruction::And || Opcode == Instruction::Or)) {
       FindMergedConditions(BOp, Succ0MBB, Succ1MBB, BrMBB, BrMBB,
-                           BOp->getOpcode(), getEdgeWeight(BrMBB, Succ0MBB),
+                           Opcode, getEdgeWeight(BrMBB, Succ0MBB),
                            getEdgeWeight(BrMBB, Succ1MBB));
       // If the compares in later blocks need to use values not currently
       // exported from this block, export them now.  This block should always
       // be the first entry.
       assert(SwitchCases[0].ThisBB == BrMBB && "Unexpected lowering!");
 
       // Allow some cases to be rejected.
       if (ShouldEmitAsBranches(SwitchCases)) {

llvm/trunk/test/CodeGen/X86/or-branch.ll

-; RUN: llc < %s -mtriple=i386-unknown-unknown -jump-is-expensive=0 | FileCheck %s --check-prefix=JUMP2
-; RUN: llc < %s -mtriple=i386-unknown-unknown -jump-is-expensive=1 | FileCheck %s --check-prefix=JUMP1
+; RUN: llc < %s -mtriple=i386-unknown-unknown -jump-is-expensive=0 | FileCheck %s --check-prefix=JUMP2 --check-prefix=CHECK
+; RUN: llc < %s -mtriple=i386-unknown-unknown -jump-is-expensive=1 | FileCheck %s --check-prefix=JUMP1 --check-prefix=CHECK
 
 define void @foo(i32 %X, i32 %Y, i32 %Z) nounwind {
 ; JUMP2-LABEL: foo:
 ; JUMP2-DAG:     jl
 ; JUMP2-DAG:     je
 ;
 ; JUMP1-LABEL: foo:
 ; JUMP1-DAG:     sete

 cond_true:
   %tmp5 = tail call i32 (...) @bar( )
   ret void
 
 UnifiedReturnBlock:
   ret void
 }
 
+; If the branch is unpredictable, don't add another branch
+; regardless of whether they are expensive or not.
+
+define void @unpredictable(i32 %X, i32 %Y, i32 %Z) nounwind {
+; CHECK-LABEL: unpredictable:
+; CHECK-DAG:     sete
+; CHECK-DAG:     setl
+; CHECK:         orb
+; CHECK:         jne
+entry:
+  %tmp1 = icmp eq i32 %X, 0
+  %tmp3 = icmp slt i32 %Y, 5
+  %tmp4 = or i1 %tmp3, %tmp1
+  br i1 %tmp4, label %cond_true, label %UnifiedReturnBlock, !unpredictable !0
+
+cond_true:
+  %tmp5 = tail call i32 (...) @bar( )
+  ret void
+
+UnifiedReturnBlock:
+  ret void
+}
+
 declare i32 @bar(...)
+
+!0 = !{}
+