Compiler/TMS320F28377D: Compiler does not emit conditional moves of floating-point registers

Part Number: TMS320F28377D

Tool/software: TI C/C++ Compiler

The C28x+FPU supports conditional moves of floating-point registers via the following instructions:

MOV32 RaH, RbH {, CNDF}

MOV32 RaH, mem32 {, CNDF}

MOV32 mem32, RbH {, CNDF}

The compiler (v18.12.1.LTS) will not emit these instructions.

For example, the code:

void fp_conditional_move(float *a, float b)
{
    *a = ( b == .5f ) ? -.5f : b;
}

generates the following assembly (-O2, -O3, and -O4)(trimmed for brevity):

||fp_conditional_move||:

        ADDB      SP,#2                 ; [CPU_ARAU]
        CMPF32    R0H,#16128            ; [CPU_FPU] |3|
        MOVST0    ZF, NF                ; [CPU_FPU] |3|
        B         ||$C$L1||,NEQ         ; [CPU_ALU] |3|
        MOVIZ     R0H,#48896            ; [CPU_FPU] |3|
||$C$L1||:   
        SUBB      SP,#2                 ; [CPU_ARAU]
        MOV32     *+XAR4[0],R0H         ; [CPU_FPU] |3|
        LRETR     ; [CPU_ALU]

Including the superfluous (?) stack pointer operations, this will typically take 12 cycles. I'm assuming the branch doesn't cause a prefetch miss which would add additional Flash wait-state cycles.

This looks like inefficient code to me. I expected a conditional overwrite of the R0H register with the value -0.5f, i.e. (my comments):

||fp_conditional_move||:
        ; *XAR4 = (R0H == .5f) ? -.5f : R0H
        MOVIZ     R1H,#48896            ; R1H = -.5f (R1H is caller-saved, i.e. trash)
        CMPF32    R0H,#16128            ; R0H == .5f (sets ZF if equal)
        MOV32     R0H, R1H, EQ          ; if (R0H == .5f) R0H = R1H
        MOV32     *+XAR4[0],R0H         ; *XAR4 = R0H
        LRETR     ; [CPU_ALU]

This code is branchless and takes 4 cycles, a speed up of 200% over the compiler-generated branching code.

Is my conditional move code correct?

Why does not compiler not emit conditional moves?

I have previously worked on C6000 cores where the compiler is very happy to emit conditional instructions.