[PATCH] D28337: [InstCombine] if the condition of a select is known, eliminate the select
Sanjay Patel via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Fri Jan 13 09:13:47 PST 2017
This revision was automatically updated to reflect the committed changes.
Closed by commit rL291915: [InstCombine] if the condition of a select may be known via assumes, eliminate… (authored by spatel).
Changed prior to commit:
https://reviews.llvm.org/D28337?vs=83757&id=84323#toc
Repository:
rL LLVM
https://reviews.llvm.org/D28337
Files:
llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
llvm/trunk/test/Transforms/InstCombine/select.ll
llvm/trunk/test/Transforms/InstSimplify/select.ll
Index: llvm/trunk/test/Transforms/InstCombine/select.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/select.ll
+++ llvm/trunk/test/Transforms/InstCombine/select.ll
@@ -1332,3 +1332,30 @@
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
+}
+
Index: llvm/trunk/test/Transforms/InstSimplify/select.ll
===================================================================
--- llvm/trunk/test/Transforms/InstSimplify/select.ll
+++ llvm/trunk/test/Transforms/InstSimplify/select.ll
@@ -402,7 +402,8 @@
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)
Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1450,6 +1450,20 @@
}
}
+ // 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;
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