Teach InstCombine to work with smaller legal types in icmp (shl %v, C1), C2

It enables to work with a smaller constant, which is target friendly for those which can compare to immediates.
It also avoids inserting a shift in favor of a trunc, which can be free on some targets.

This used to work until LLVM-3.1, but regressed with the 3.2 release.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175270 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 61 insertions, 3 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 40e559eda5..2e7bd922a2 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1331,6 +1331,25 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
       return new ICmpInst(TrueIfSigned ? ICmpInst::ICMP_NE : ICmpInst::ICMP_EQ,
                           And, Constant::getNullValue(And->getType()));
     }
+
+    // Transform (icmp pred iM (shl iM %v, N), CI)
+    // -> (icmp pred i(M-N) (trunc %v iM to i(N-N)), (trunc (CI>>N))
+    // Transform the shl to a trunc if (trunc (CI>>N)) has no loss.
+    // This enables to get rid of the shift in favor of a trunc which can be
+    // free on the target. It has the additional benefit of comparing to a
+    // smaller constant, which will be target friendly.
+    unsigned Amt = ShAmt->getLimitedValue(TypeBits-1);
+    if (Amt != 0 && RHSV.countTrailingZeros() >= Amt) {
+      Type *NTy = IntegerType::get(ICI.getContext(), TypeBits - Amt);
+      Constant *NCI = ConstantExpr::getTrunc(
+                        ConstantExpr::getAShr(RHS,
+                          ConstantInt::get(RHS->getType(), Amt)),
+                        NTy);
+      return new ICmpInst(ICI.getPredicate(),
+                          Builder->CreateTrunc(LHSI->getOperand(0), NTy),
+                          ConstantExpr::getTrunc(NCI, NTy));
+    }
+
     break;
   }
 
diff --git a/test/Transforms/InstCombine/exact.ll b/test/Transforms/InstCombine/exact.ll
index 14741e3c1c..88ca88c3b9 100644
--- a/test/Transforms/InstCombine/exact.ll
+++ b/test/Transforms/InstCombine/exact.ll
@@ -99,9 +99,9 @@ define i1 @ashr_icmp2(i64 %X) nounwind {
 ; PR9998
 ; Make sure we don't transform the ashr here into an sdiv
 ; CHECK: @pr9998
-; CHECK: = and i32 %V, 1
-; CHECK: %Z = icmp ne
-; CHECK: ret i1 %Z
+; CHECK:      [[BIT:%[A-Za-z0-9.]+]] = and i32 %V, 1
+; CHECK-NEXT: [[CMP:%[A-Za-z0-9.]+]] = icmp ne i32 [[BIT]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
 define i1 @pr9998(i32 %V) nounwind {
 entry:
   %W = shl i32 %V, 31
@@ -112,6 +112,7 @@ entry:
 }
 
 
+
 ; CHECK: @udiv_icmp1
 ; CHECK: icmp ne i64 %X, 0
 define i1 @udiv_icmp1(i64 %X) nounwind {
diff --git a/test/Transforms/InstCombine/icmp.ll b/test/Transforms/InstCombine/icmp.ll
index 8fb6144c3f..331eb3f21a 100644
--- a/test/Transforms/InstCombine/icmp.ll
+++ b/test/Transforms/InstCombine/icmp.ll
@@ -706,3 +706,41 @@ define i1 @test69(i32 %c) nounwind uwtable {
   %3 = or i1 %1, %2
   ret i1 %3
 }
+
+; CHECK: @icmp_sext16trunc
+; CHECK-NEXT: %1 = trunc i32 %x to i16
+; CHECK-NEXT: %cmp = icmp slt i16 %1, 36
+define i1 @icmp_sext16trunc(i32 %x) {
+  %trunc = trunc i32 %x to i16
+  %sext = sext i16 %trunc to i32
+  %cmp = icmp slt i32 %sext, 36
+  ret i1 %cmp
+}
+
+; CHECK: @icmp_sext8trunc
+; CHECK-NEXT: %1 = trunc i32 %x to i8
+; CHECK-NEXT: %cmp = icmp slt i8 %1, 36
+define i1 @icmp_sext8trunc(i32 %x) {
+  %trunc = trunc i32 %x to i8
+  %sext = sext i8 %trunc to i32
+  %cmp = icmp slt i32 %sext, 36
+  ret i1 %cmp
+}
+
+; CHECK: @icmp_shl16
+; CHECK-NEXT: %1 = trunc i32 %x to i16
+; CHECK-NEXT: %cmp = icmp slt i16 %1, 36
+define i1 @icmp_shl16(i32 %x) {
+  %shl = shl i32 %x, 16
+  %cmp = icmp slt i32 %shl, 2359296
+  ret i1 %cmp
+}
+
+; CHECK: @icmp_shl24
+; CHECK-NEXT: %1 = trunc i32 %x to i8
+; CHECK-NEXT: %cmp = icmp slt i8 %1, 36
+define i1 @icmp_shl24(i32 %x) {
+  %shl = shl i32 %x, 24
+  %cmp = icmp slt i32 %shl, 603979776
+  ret i1 %cmp
+}