[PATCH] D28337: [InstSimplify] if the condition of a select is known, eliminate the select

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: efriedma, hfinkel, majnemer.
spatel added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

This is a generic (and potentially expensive) solution for PR31512:
https://llvm.org/bugs/show_bug.cgi?id=31512

The motivation is that we will need to increase usage of llvm.assume and/or metadata to solve PR28430:
https://llvm.org/bugs/show_bug.cgi?id=28430

...and this kind of simplification is needed to take advantage of that extra information.

A couple of things I learned while looking at this:

1. We already have a call to computeKnownBits() at the end of llvm::SimplifyInstruction(), so this case already works: define i8 @assume_sel_cond4(i1 %cond, i8 %x, i8 %y) { call void @llvm.assume(i1 %cond) %notcond = xor i1 %cond, 1   <--- known false via computeKnownBits %sel = select i1 %notcond, i8 %x, i8 %y ret i8 %sel }

2. The test cases in this patch will not work with 'opt -instcombine' even though instcombine always calls SimplifySelectInst() before it tries other transforms. This is because in InstCombiner::visitSelectInst(): if (Value *V = SimplifySelectInst(CondVal, TrueVal, FalseVal, DL, &TLI, &DT, &AC))

...we don't include the select instruction as the context instruction. I don't think that's intentional, but that is true for many of the Simplify* calls in InstCombiner. As a counter example, we do provide the context instruction when InstCombiner::visitICmpInst() calls SimplifyICmpInst().


https://reviews.llvm.org/D28337

Files:
  lib/Analysis/InstructionSimplify.cpp
  test/Transforms/InstSimplify/select.ll


Index: test/Transforms/InstSimplify/select.ll
===================================================================
--- test/Transforms/InstSimplify/select.ll
+++ test/Transforms/InstSimplify/select.ll
@@ -402,15 +402,14 @@
   ret i32* %sel
 }
 
-; FIXME: If the condition is known, we don't need to select.
+; If the condition is known, we don't need to select.
 
 declare void @llvm.assume(i1)
 
 define i8 @assume_sel_cond(i1 %cond, i8 %x, i8 %y) {
 ; CHECK-LABEL: @assume_sel_cond(
 ; CHECK-NEXT:    call void @llvm.assume(i1 %cond)
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 %cond, i8 %x, i8 %y
-; CHECK-NEXT:    ret i8 [[SEL]]
+; CHECK-NEXT:    ret i8 %x
 ;
   call void @llvm.assume(i1 %cond)
   %sel = select i1 %cond, i8 %x, i8 %y
@@ -421,8 +420,7 @@
 ; CHECK-LABEL: @do_not_assume_sel_cond(
 ; CHECK-NEXT:    [[NOTCOND:%.*]] = icmp eq i1 %cond, false
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[NOTCOND]])
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 %cond, i8 %x, i8 %y
-; CHECK-NEXT:    ret i8 [[SEL]]
+; CHECK-NEXT:    ret i8 %y
 ;
   %notcond = icmp eq i1 %cond, false
   call void @llvm.assume(i1 %notcond)
Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -3708,6 +3708,16 @@
           simplifySelectWithICmpCond(CondVal, TrueVal, FalseVal, Q, MaxRecurse))
     return V;
 
+  // If we can compute the condition, there's no need for a select.
+  if (CondVal->getType()->getPrimitiveSizeInBits() == 1) {
+    APInt KnownOne(1, 0), KnownZero(1, 0);
+    computeKnownBits(CondVal, KnownZero, KnownOne, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
+    if (KnownOne == 1)
+      return TrueVal;
+    if (KnownZero == 1)
+      return FalseVal;
+  }
+
   return nullptr;
 }
 


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