[IndVars] Canonicalize comparisons between non-negative values and in…

…dvars

-If there is a IndVar which is known to be non-negative, and there is a value which is also non-negative,
then signed and unsigned comparisons between them produce the same result. Both of those can be
seen in the same loop. To allow other optimizations to simplify them, we turn all instructions like

  %c = icmp slt i32 %iv, %b
to

  %c = icmp ult i32 %iv, %b

if both %iv and %b are known to be non-negative.

Differential Revision: https://reviews.llvm.org/D34979

llvm-svn: 307126

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -262,6 +262,10 @@ void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
     ICmp->setPredicate(InvariantPredicate);
     ICmp->setOperand(0, NewLHS);
     ICmp->setOperand(1, NewRHS);
+  } else if (ICmpInst::isSigned(Pred) &&
+             SE->isKnownNonNegative(S) && SE->isKnownNonNegative(X)) {
+    DEBUG(dbgs() << "INDVARS: Turn to unsigned comparison: " << *ICmp << '\n');
+    ICmp->setPredicate(ICmpInst::getUnsignedPredicate(Pred));
   } else
     return;
 

llvm/test/Analysis/ScalarEvolution/guards.ll

@@ -19,7 +19,7 @@ entry:
 loop:
 ; CHECK: loop:
 ; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
-; CHECK:  %iv.inc.cmp = icmp slt i32 %iv.inc, %len
+; CHECK:  %iv.inc.cmp = icmp ult i32 %iv.inc, %len
 ; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 %iv.inc.cmp) [ "deopt"() ]
 ; CHECK: leave:
 
@@ -41,7 +41,7 @@ leave:
 
 define void @test_2(i32 %n, i32* %len_buf) {
 ; CHECK-LABEL: @test_2(
-; CHECK:  [[LEN_SEXT:%[^ ]+]] = sext i32 %len to i64
+; CHECK:  [[LEN_ZEXT:%[^ ]+]] = zext i32 %len to i64
 ; CHECK:  br label %loop
 
 entry:
@@ -52,7 +52,7 @@ loop:
 ; CHECK: loop:
 ; CHECK:  %indvars.iv = phi i64 [ %indvars.iv.next, %loop ], [ 0, %entry ]
 ; CHECK:  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
-; CHECK:  %iv.inc.cmp = icmp slt i64 %indvars.iv.next, [[LEN_SEXT]]
+; CHECK:  %iv.inc.cmp = icmp ult i64 %indvars.iv.next, [[LEN_ZEXT]]
 ; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 %iv.inc.cmp) [ "deopt"() ]
 ; CHECK: leave:
 

llvm/test/Transforms/IndVarSimplify/canonicalize-cmp.ll

@@ -0,0 +1,52 @@
+; RUN: opt -S -indvars < %s | FileCheck %s
+
+; Check that we replace signed comparisons between non-negative values with
+; unsigned comparisons if we can.
+
+target datalayout = "n8:16:32:64"
+
+define i32 @test_01(i32 %a, i32 %b, i32* %p) {
+
+; CHECK-LABEL: @test_01(
+; CHECK-NOT:   icmp slt
+; CHECK:       %cmp1 = icmp ult i32 %iv, 100
+; CHECK:       %cmp2 = icmp ult i32 %iv, 100
+; CHECK-NOT:   %cmp3
+; CHECK:       %exitcond = icmp ne i32 %iv.next, 1000
+
+entry:
+  br label %loop.entry
+
+loop.entry:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop.be ]
+  %cmp1 = icmp slt i32 %iv, 100
+  br i1 %cmp1, label %b1, label %b2
+
+b1:
+  store i32 %iv, i32* %p
+  br label %merge
+
+b2:
+  store i32 %a, i32* %p
+  br label %merge
+
+merge:
+  %cmp2 = icmp ult i32 %iv, 100
+  br i1 %cmp2, label %b3, label %b4
+
+b3:
+  store i32 %iv, i32* %p
+  br label %loop.be
+
+b4:
+  store i32 %b, i32* %p
+  br label %loop.be
+
+loop.be:
+  %iv.next = add i32 %iv, 1
+  %cmp3 = icmp slt i32 %iv.next, 1000
+  br i1 %cmp3, label %loop.entry, label %exit
+
+exit:
+  ret i32 %iv
+}

llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll

@@ -111,7 +111,7 @@ return:
 ; Indvars should not turn the second loop into an infinite one.
 
 ; CHECK-LABEL: @func_11(
-; CHECK: %tmp5 = icmp slt i32 %__key6.0, 10
+; CHECK: %tmp5 = icmp ult i32 %__key6.0, 10
 ; CHECK-NOT: br i1 true, label %noassert68, label %unrolledend
 
 define i32 @func_11() nounwind uwtable {
@@ -163,7 +163,7 @@ declare void @llvm.trap() noreturn nounwind
 
 ; In this case the second loop only has a single iteration, fold the header away
 ; CHECK-LABEL: @func_12(
-; CHECK: %tmp5 = icmp slt i32 %__key6.0, 10
+; CHECK: %tmp5 = icmp ult i32 %__key6.0, 10
 ; CHECK: br i1 true, label %noassert68, label %unrolledend
 define i32 @func_12() nounwind uwtable {
 entry:

llvm/test/Transforms/IndVarSimplify/widen-loop-comp.ll

@@ -64,7 +64,7 @@ for.end:
 ; CHECK-LABEL: @test2
 ; CHECK: for.body4.us
 ; CHECK: %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
-; CHECK: %cmp2.us = icmp slt i64
+; CHECK: %cmp2.us = icmp ult i64
 ; CHECK-NOT: %2 = trunc i64 %indvars.iv.next to i32
 ; CHECK-NOT: %cmp2.us = icmp slt i32