[InstCombine] Gating select arithmetic optimization

[mcberg2017]

Summary

These changes faciliate positive behavior for arithmetic based select expressions that match its translation criteria, keeping code size gated to neutral or improved scenarios.

[mcrosier]

Please use the -U option to include full context in your diffs (e.g., git show HEAD -U999999 > mypatch.patch).

[mcberg2017]

Updated for full context diff, the patch file now contains all the code.

[mcrosier]

LGTM, but I'll defer to David and Quentin for final approval.

[mcberg2017]

Chad, would like me to amend the test file to include the changes you and
Balaram made?

[majnemer]

LGTM

[mcrosier]

In D38263#882286, @mcberg2017 wrote:

Chad, would like me to amend the test file to include the changes you and
Balaram made?

I think it would be a good idea to add both test cases as it makes it easier to understand this change, IMHO.

[mcrosier]

Do you have commit access, btw? I can commit if not.

[mcrosier]

In D38263#882297, @mcrosier wrote:

Do you have commit access, btw? I can commit if not.

Nevermind, I just went ahead and committed the change. Thanks for the fix!

lib/Transforms/InstCombine/InstructionCombining.cpp

   Value *SI = nullptr;
   if (match(LHS, m_Select(m_Value(A), m_Value(B), m_Value(C))) &&
       match(RHS, m_Select(m_Specific(A), m_Value(D), m_Value(E)))) {
+    bool SelectsHaveOneUse = LHS->hasOneUse() && RHS->hasOneUse();
     BuilderTy::FastMathFlagGuard Guard(Builder);
     if (isa<FPMathOperator>(&I))
       Builder.setFastMathFlags(I.getFastMathFlags());

     Value *V2 = SimplifyBinOp(Opcode, B, D, SQ.getWithInstruction(&I));
     if (V1 && V2)
       SI = Builder.CreateSelect(A, V2, V1);
-    else if (V2)
+    else if (V2 && SelectsHaveOneUse)
       SI = Builder.CreateSelect(A, V2, Builder.CreateBinOp(Opcode, C, E));
-    else if (V1)
+    else if (V1 && SelectsHaveOneUse)
       SI = Builder.CreateSelect(A, Builder.CreateBinOp(Opcode, B, D), V1);
 
     if (SI)

test/Transforms/InstCombine/select_arithmetic.ll

 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 
 ; Tests folding constants from two similar selects that feed a add
-define float @test1(i1 zeroext %arg) #0 {
+define float @test1a(i1 zeroext %arg) #0 {
   %tmp = select i1 %arg, float 5.000000e+00, float 6.000000e+00
   %tmp1 = select i1 %arg, float 1.000000e+00, float 9.000000e+00
   %tmp2 = fadd float %tmp, %tmp1
   ret float %tmp2
-; CHECK-LABEL: @test1(
+; CHECK-LABEL: @test1a(
 ; CHECK: %tmp2 = select i1 %arg, float 6.000000e+00, float 1.500000e+01
 ; CHECK-NOT: fadd
 ; CHECK: ret float %tmp2
 }
 
+; Tests folding multiple expression constants from similar selects that feed a adds
+define float @test1b(i1 zeroext %arg) #0 {
+  %tmp = select i1 %arg, float 5.000000e+00, float 6.000000e+00
+  %tmp1 = select i1 %arg, float 1.000000e+00, float 9.000000e+00
+  %tmp2 = select i1 %arg, float 2.500000e-01, float 4.000000e+00
+  %tmp3 = fadd float %tmp, %tmp1
+  %tmp4 = fadd float %tmp2, %tmp1
+  %tmp5 = fadd float %tmp4, %tmp3
+  ret float %tmp5
+; CHECK-LABEL: @test1b(
+; CHECK: %tmp5 = select i1 %arg, float 7.250000e+00, float 2.800000e+01
+; CHECK-NOT: fadd
+; CHECK: ret float %tmp5
+}
+
 ; Tests folding constants from two similar selects that feed a sub
 define float @test2(i1 zeroext %arg) #0 {
   %tmp = select i1 %arg, float 5.000000e+00, float 6.000000e+00