[Thumb1] Fix cost calculation for complemented immediates

Materializing something like "-3" can be done as 2 instructions:
  MOV r0, #3
  MVN r0, r0

This has a cost of 2, not 3. It looks like we were already trying to detect this pattern in TII::getIntImmCost(), but were taking the complement of the zero-extended value instead of the sign-extended value which is unlikely to ever produce a number < 256.

There were no tests failing after changing this... :/

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280928 91177308-0d34-0410-b5e6-96231b3b80d8

Author: James Molloy

lib/Target/ARM/ARMTargetTransformInfo.cpp

@@ -41,7 +41,7 @@ int ARMTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty) {
   // Thumb1.
   if (SImmVal >= 0 && SImmVal < 256)
     return 1;
-  if ((~ZImmVal < 256) || ARM_AM::isThumbImmShiftedVal(ZImmVal))
+  if ((~SImmVal < 256) || ARM_AM::isThumbImmShiftedVal(ZImmVal))
     return 2;
   // Load from constantpool.
   return 3;

test/CodeGen/ARM/immcost.ll

@@ -0,0 +1,21 @@
+; RUN: llc %s -o - -O1 -debug-only=consthoist 2>&1 | FileCheck %s
+; REQUIRES: asserts
+
+target datalayout = "e-m:o-p:32:32-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
+target triple = "thumbv6m-apple-ios8.0.0"
+
+declare void @g(i32)
+
+; CHECK: Collect constant i32 -3 from   call void @g(i32 -3) with cost 2
+define void @f(i1 %cond) {
+entry:
+  call void @g(i32 -3)
+  br i1 %cond, label %true, label %ret
+
+true:
+  call void @g(i32 -3)
+  br label %ret
+
+ret:
+  ret void
+}