[SystemZ] Clear NW flags on an ISD::SUB when reused as comparison.

The SystemZ backend will try to reuse an existing subtraction of two values
whenever they are to be compared for equality. This depends on the SystemZ
subtraction instruction setting the condition code, which can also signal
overflow.

A later pass will remove the compare and reuse the CC from the subtraction
directly. However, if that subtraction has the NSW flag set it will not
include the overflow bit in the updated CC user. That was a bug which can
lead to wrong results, as shown by a csmith program.

Fixes: https://github.com/llvm/llvm-project/issues/61268

Reviewed By: nikic, uweigand

Differential Revision: https://reviews.llvm.org/D145811

diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
index 04ba0db..3508194 100644 (file)
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -2423,6 +2423,12 @@ static void adjustForSubtraction(SelectionDAG &DAG, const SDLoc &DL,
       if (N->getOpcode() == ISD::SUB &&
           ((N->getOperand(0) == C.Op0 && N->getOperand(1) == C.Op1) ||
            (N->getOperand(0) == C.Op1 && N->getOperand(1) == C.Op0))) {
+        // Disable the nsw and nuw flags: the backend needs to handle
+        // overflow as well during comparison elimination.
+        SDNodeFlags Flags = N->getFlags();
+        Flags.setNoSignedWrap(false);
+        Flags.setNoUnsignedWrap(false);
+        N->setFlags(Flags);
         C.Op0 = SDValue(N, 0);
         C.Op1 = DAG.getConstant(0, DL, N->getValueType(0));
         return;

diff --git a/llvm/test/CodeGen/SystemZ/int-cmp-62.ll b/llvm/test/CodeGen/SystemZ/int-cmp-62.ll
new file mode 100644 (file)
index 0000000..c57cf57
--- /dev/null
+++ b/llvm/test/CodeGen/SystemZ/int-cmp-62.ll
@@ -0,0 +1,39 @@
+; RUN: llc < %s -mtriple=s390x-linux-gnu -mcpu=z15 | FileCheck %s
+;
+; Test that a CC result of a sub that can overflow is tested with the right predicate.
+
+define i32 @fun0(i32 %a, i32 %b, ptr %dest) {
+; CHECK-LABEL: fun0
+; CHECK: s %r2, 0(%r4)
+; CHECK: bner %r14
+entry:
+  %cur = load i32, ptr %dest
+  %res = sub nsw i32 %a, %cur
+  %cmp = icmp ne i32 %a, %cur
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 %b, ptr %dest
+  br label %exit
+
+exit:
+  ret i32 %res
+}
+
+define i32 @fun1(i32 %a, i32 %b, ptr %dest) {
+; CHECK-LABEL: fun1
+; CHECK: s %r2, 0(%r4)
+; CHECK: bner %r14
+entry:
+  %cur = load i32, ptr %dest
+  %res = sub nuw i32 %a, %cur
+  %cmp = icmp ne i32 %a, %cur
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 %b, ptr %dest
+  br label %exit
+
+exit:
+  ret i32 %res
+}