[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.

llvm-svn: 259381

2 files changed, 27 insertions, 1 deletions

diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
index ee732675fb3..b0a61276463 100644
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -1849,7 +1849,7 @@ static unsigned getTestUnderMaskCond(unsigned BitSize, unsigned CCMask,
     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)
diff --git a/llvm/test/CodeGen/SystemZ/int-cmp-53.ll b/llvm/test/CodeGen/SystemZ/int-cmp-53.ll
new file mode 100644
index 00000000000..b7d985eeefe
--- /dev/null
+++ b/llvm/test/CodeGen/SystemZ/int-cmp-53.ll
@@ -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
+}
+