[llvm] r291915 - [InstCombine] if the condition of a select may be known via assumes, eliminate the select

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Fri Jan 13 11:02:42 2017
New Revision: 291915

URL: http://llvm.org/viewvc/llvm-project?rev=291915&view=rev
Log:
[InstCombine] if the condition of a select may be known via assumes, eliminate the select

This is a limited 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.

The 'not' test case would be handled by:
https://reviews.llvm.org/D28485

Differential Revision:
https://reviews.llvm.org/D28337

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
    llvm/trunk/test/Transforms/InstCombine/select.ll
    llvm/trunk/test/Transforms/InstSimplify/select.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp?rev=291915&r1=291914&r2=291915&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp Fri Jan 13 11:02:42 2017
@@ -1450,6 +1450,20 @@ Instruction *InstCombiner::visitSelectIn
     }
   }
 
+  // If we can compute the condition, there's no need for a select.
+  // Like the above fold, we are attempting to reduce compile-time cost by
+  // putting this fold here with limitations rather than in InstSimplify.
+  // The motivation for this call into value tracking is to take advantage of
+  // the assumption cache, so make sure that is populated.
+  if (!CondVal->getType()->isVectorTy() && !AC.assumptions().empty()) {
+    APInt KnownOne(1, 0), KnownZero(1, 0);
+    computeKnownBits(CondVal, KnownZero, KnownOne, 0, &SI);
+    if (KnownOne == 1)
+      return replaceInstUsesWith(SI, TrueVal);
+    if (KnownZero == 1)
+      return replaceInstUsesWith(SI, FalseVal);
+  }
+
   if (Instruction *BitCastSel = foldSelectCmpBitcasts(SI, *Builder))
     return BitCastSel;
 

Modified: llvm/trunk/test/Transforms/InstCombine/select.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/select.ll?rev=291915&r1=291914&r2=291915&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/select.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/select.ll Fri Jan 13 11:02:42 2017
@@ -1332,3 +1332,30 @@ define <4 x i32> @cannot_canonicalize_to
   ret <4 x i32> %sel
 }
 
+declare void @llvm.assume(i1)
+
+define i8 @assume_cond_true(i1 %cond, i8 %x, i8 %y) {
+; CHECK-LABEL: @assume_cond_true(
+; CHECK-NEXT:    call void @llvm.assume(i1 %cond)
+; CHECK-NEXT:    ret i8 %x
+;
+  call void @llvm.assume(i1 %cond)
+  %sel = select i1 %cond, i8 %x, i8 %y
+  ret i8 %sel
+}
+
+; FIXME: computeKnownBitsFromAssume() should understand the 'not' of an assumed condition.
+
+define i8 @assume_cond_false(i1 %cond, i8 %x, i8 %y) {
+; CHECK-LABEL: @assume_cond_false(
+; CHECK-NEXT:    [[NOTCOND:%.*]] = xor i1 %cond, true
+; CHECK-NEXT:    call void @llvm.assume(i1 [[NOTCOND]])
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 %cond, i8 %x, i8 %y
+; CHECK-NEXT:    ret i8 [[SEL]]
+;
+  %notcond = xor i1 %cond, true
+  call void @llvm.assume(i1 %notcond)
+  %sel = select i1 %cond, i8 %x, i8 %y
+  ret i8 %sel
+}
+

Modified: llvm/trunk/test/Transforms/InstSimplify/select.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/select.ll?rev=291915&r1=291914&r2=291915&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/select.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/select.ll Fri Jan 13 11:02:42 2017
@@ -402,7 +402,8 @@ define i32* @select_icmp_pointers(i32* %
   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, but we're not
+; doing this fold here to avoid compile-time cost.
 
 declare void @llvm.assume(i1)