sgt on i1s also encodes implication

[reames]

Summary

Follow on to http://reviews.llvm.org/D13074, implementing something pointed out by Sanjoy. His truth table from his comment on that bug summarizes things well:
LHS | RHS | LHS >=s RHS | LHS implies RHS
0 | 0 | 1 (0 >= 0) | 1
0 | 1 | 1 (0 >= -1) | 1
1 | 0 | 0 (-1 >= 0) | 0
1 | 1 | 1 (-1 >= -1) | 1

The key point is that an "i1 1" is the value "-1", not "1".

[sanjoy]

The change itself is fine, but I think the comments on what inequalities are equivalent to implication should not be on implies itself; but on the code that simplifies inequalities as implication.

llvm/trunk/lib/Analysis/InstructionSimplify.cpp

case ICmpInst::ICMP_UGT:
       break;
     case ICmpInst::ICMP_UGE:
       // X >=u 1 -> X
       if (match(RHS, m_One()))
         return LHS;
       if (isImpliedCondition(RHS, LHS))
         return getTrue(ITy);
       break;
+    case ICmpInst::ICMP_SGE:
+      /// For signed comparison, the values for an i1 are 0 and -1 
+      /// respectively. This maps into a truth table of:
+      /// LHS | RHS | LHS >=s RHS   | LHS implies RHS
+      ///  0  |  0  |  1 (0 >= 0)   |  1
+      ///  0  |  1  |  1 (0 >= -1)  |  1
+      ///  1  |  0  |  0 (-1 >= 0)  |  0
+      ///  1  |  1  |  1 (-1 >= -1) |  1
+      if (isImpliedCondition(LHS, RHS))
+        return getTrue(ITy);
+      break;
     case ICmpInst::ICMP_SLT:
       // X <s 0 -> X
       if (match(RHS, m_Zero()))
         return LHS;
       break;
     case ICmpInst::ICMP_SLE:
       // X <=s -1 -> X
       if (match(RHS, m_One()))

llvm/trunk/test/Transforms/InstSimplify/implies.ll

 
 ; Don't crash on vector inputs - pr25040
 define <4 x i1> @test6(<4 x i1> %a, <4 x i1> %b) {
 ; CHECK-LABEL: @test6
 ; CHECK: ret <4 x i1> %res
   %res = icmp ule <4 x i1> %a, %b
   ret <4 x i1> %res
 }
+
+; X >=(s) Y == X ==> Y (i1 1 becomes -1 for reasoning)
+define i1 @test_sge(i32 %length.i, i32 %i) {
+; CHECK-LABEL: @test_sge
+; CHECK: ret i1 true
+  %iplus1 = add nsw nuw i32 %i, 1
+  %var29 = icmp ult i32 %i, %length.i
+  %var30 = icmp ult i32 %iplus1, %length.i
+  %res = icmp sge i1 %var30, %var29
+  ret i1 %res
+}