Allow negative immediates in ARM and Thumb2 compares.

ARM and Thumb2 mode can use cmn instructions to compare against negative
immediates. Thumb1 mode can't.

llvm-svn: 154183

2 files changed, 37 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index fab3e24551b..c20ee674ce8 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -8614,10 +8614,12 @@ bool ARMTargetLowering::isLegalAddressingMode(const AddrMode &AM,
 /// a register against the immediate without having to materialize the
 /// immediate into a register.
 bool ARMTargetLowering::isLegalICmpImmediate(int64_t Imm) const {
+  // Thumb2 and ARM modes can use cmn for negative immediates.
   if (!Subtarget->isThumb())
-    return ARM_AM::getSOImmVal(Imm) != -1;
+    return ARM_AM::getSOImmVal(std::abs(Imm)) != -1;
   if (Subtarget->isThumb2())
-    return ARM_AM::getT2SOImmVal(Imm) != -1;
+    return ARM_AM::getT2SOImmVal(std::abs(Imm)) != -1;
+  // Thumb1 doesn't have cmn, and only 8-bit immediates.
   return Imm >= 0 && Imm <= 255;
 }
 
diff --git a/llvm/test/CodeGen/ARM/lsr-icmp-imm.ll b/llvm/test/CodeGen/ARM/lsr-icmp-imm.ll
new file mode 100644
index 00000000000..5283f5747d9
--- /dev/null
+++ b/llvm/test/CodeGen/ARM/lsr-icmp-imm.ll
@@ -0,0 +1,33 @@
+; RUN: llc -mtriple=thumbv7-apple-ios -disable-code-place < %s | FileCheck %s
+; RUN: llc -mtriple=armv7-apple-ios   -disable-code-place < %s | FileCheck %s
+
+; LSR should compare against the post-incremented induction variable.
+; In this case, the immediate value is -2 which requires a cmn instruction.
+;
+; CHECK: f:
+; CHECK: %for.body
+; CHECK: sub{{.*}}[[IV:r[0-9]+]], #2
+; CHECK: cmn{{.*}}[[IV]], #2
+; CHECK: bne
+define i32 @f(i32* nocapture %a, i32 %i) nounwind readonly ssp {
+entry:
+  %cmp3 = icmp eq i32 %i, -2
+  br i1 %cmp3, label %for.end, label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %bi.06 = phi i32 [ %i.addr.0.bi.0, %for.body ], [ 0, %entry ]
+  %i.addr.05 = phi i32 [ %sub, %for.body ], [ %i, %entry ]
+  %b.04 = phi i32 [ %.b.0, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %a, i32 %i.addr.05
+  %0 = load i32* %arrayidx, align 4
+  %cmp1 = icmp sgt i32 %0, %b.04
+  %.b.0 = select i1 %cmp1, i32 %0, i32 %b.04
+  %i.addr.0.bi.0 = select i1 %cmp1, i32 %i.addr.05, i32 %bi.06
+  %sub = add nsw i32 %i.addr.05, -2
+  %cmp = icmp eq i32 %i.addr.05, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  %bi.0.lcssa = phi i32 [ 0, %entry ], [ %i.addr.0.bi.0, %for.body ]
+  ret i32 %bi.0.lcssa
+}