[llvm] r259381 - [SystemZ] Fix wrong-code generation for certain always-false conditions

[Ulrich Weigand via llvm-commits]

Author: uweigand
Date: Mon Feb  1 12:31:19 2016
New Revision: 259381

URL: http://llvm.org/viewvc/llvm-project?rev=259381&view=rev
Log:
[SystemZ] Fix wrong-code generation for certain always-false conditions

We've found another bug in the code generation logic conditions for a
certain class of always-false conditions, those of the form
   if ((a & 1) < 0)

These only reach the back end when compiling without optimization.

The bug was introduced by the choice of using TEST UNDER MASK
to implement a check for
   if ((a & MASK) < VAL)
as
   if ((a & MASK) == 0)

where VAL is less than the the lowest bit of MASK.  This is correct
in all cases except for VAL == 0, in which case the original
condition is always false, but the replacement isn't.

Fixed by excluding that particular case.


Added:
    llvm/trunk/test/CodeGen/SystemZ/int-cmp-53.ll
Modified:
    llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp

Modified: llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp?rev=259381&r1=259380&r2=259381&view=diff
==============================================================================
--- llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/SystemZ/SystemZISelLowering.cpp Mon Feb  1 12:31:19 2016
@@ -1849,7 +1849,7 @@ static unsigned getTestUnderMaskCond(uns
     if (CCMask == SystemZ::CCMASK_CMP_NE)
       return SystemZ::CCMASK_TM_SOME_1;
   }
-  if (EffectivelyUnsigned && CmpVal <= Low) {
+  if (EffectivelyUnsigned && CmpVal > 0 && CmpVal <= Low) {
     if (CCMask == SystemZ::CCMASK_CMP_LT)
       return SystemZ::CCMASK_TM_ALL_0;
     if (CCMask == SystemZ::CCMASK_CMP_GE)

Added: llvm/trunk/test/CodeGen/SystemZ/int-cmp-53.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/SystemZ/int-cmp-53.ll?rev=259381&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/SystemZ/int-cmp-53.ll (added)
+++ llvm/trunk/test/CodeGen/SystemZ/int-cmp-53.ll Mon Feb  1 12:31:19 2016
@@ -0,0 +1,26 @@
+; This used to incorrectly use a TMLL for an always-false test at -O0.
+;
+; RUN: llc -O0 < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+define void @test(i8 *%input, i32 *%result) {
+entry:
+; CHECK-NOT: tmll
+
+  %0 = load i8, i8* %input, align 1
+  %1 = trunc i8 %0 to i1
+  %2 = zext i1 %1 to i32
+  %3 = icmp sge i32 %2, 0
+  br i1 %3, label %if.then, label %if.else
+
+if.then:
+  store i32 1, i32* %result, align 4
+  br label %return
+
+if.else:
+  store i32 0, i32* %result, align 4
+  br label %return
+
+return:
+  ret void
+}
+