[clang] [llvm] Add support for flag output operand "=@cc" for SystemZ. (PR #125970)

[Ulrich Weigand via llvm-commits]

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@@ -2621,9 +2621,18 @@ EmitAsmStores(CodeGenFunction &CGF, const AsmStmt &S,
     if ((i < ResultRegIsFlagReg.size()) && ResultRegIsFlagReg[i]) {
       // Target must guarantee the Value `Tmp` here is lowered to a boolean
       // value.
-      llvm::Constant *Two = llvm::ConstantInt::get(Tmp->getType(), 2);
+      // Lowering 'Tmp' as - 'icmp ult %Tmp , CCUpperBound'. On some targets
+      // CCUpperBound is not binary. CCUpperBound is 4 for SystemZ,
+      // interval [0, 4). With this range known, llvm.assume intrinsic guides
+      // optimizer to generate more optimized IR in most of the cases as
+      // observed for select_cc on SystemZ unit tests for flag output operands.
+      // For some cases for br_cc, generated IR was weird. e.g. switch table
+      // for simple simple comparison terms for br_cc.
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uweigand wrote:

You don't need to explain that wrong code will result from an incorrect assertion.

https://github.com/llvm/llvm-project/pull/125970