[llvm] [RISCV] Explicitly set FRM defs as non-dead to prevent their reordering with instructions that may use it (PR #135176)

[Craig Topper via llvm-commits]

topperc wrote:

> I think we might need a bigger hammer. This case also hoists but we don't need if @foo uses FRM or changes FRM.
> 
> ```
> define double @bar(double %0, double %1, i64 %n) strictfp {                      
> entry:                                                                           
>     br label %loop                                                               
> loop:                                                                            
>     %cnt = phi i64 [0, %entry], [%cnt_inc, %loop]                                
>     %acc = phi double [0.0, %entry], [%f2, %loop]                                
>     call void @llvm.set.rounding(i32 2) strictfp                                 
>     %f1 = call double @foo(double %acc, double %0) strictfp                      
>     call void @llvm.set.rounding(i32 1) strictfp                                 
>     %f2 = call double @foo(double %f1, double %1) strictfp                       
>     %cnt_inc = add i64 %cnt, 1                                                   
>     %cond = icmp eq i64 %cnt_inc, %n                                             
>     br i1 %cond, label %loop, label %exit                                        
> exit:                                                                            
>     ret double %f2                                                               
> }                                                                                
>                                                                                  
> declare double @foo(double, double)
> ```

Looks like this patch fixes this test case too.

https://github.com/llvm/llvm-project/pull/135176