Thumb compiler oddity

[David Boddie]

I'm using the Inferno Thumb compiler, tc, to compile Inferno (unsurprisingly)
and encountered a problem. Since 9front appears to have the same compiler, I
thought that I should mention it on both 9fans and Inferno mailing lists.

The problem is with the libmath/fdlibm/e_exp.c when compiled using tc,
5c (arm), 0c (spim), and maybe others. The relevant chunk of code is the
first branch of this:

if(k >= -1021) {
__HI(y) += (k<<20); /* add k to y's exponent */
return y;
} else {
__HI(y) += ((k+1000)<<20);/* add k to y's exponent */
return y*twom1000;
}

where __HI is defined as

#define __HI(x) *(1+(int*)&x)

for little-endian doubles.

The code generated by tc for the k >= -1021 branch is this, which I've
annotated:

MOVD F0,F3 y
CMP $-1021,R4, compare k to -1021
BLT ,9(PC) k < -1021, skip past RET below
MOVW R4,R1
SLL $20,R1 k << 20
MOVW y-4(SP),R2 load the high word of y in memory
ADD R1,R2 add k << 20 to it
MOVW R2,y-4(SP) store the result back to memory
MOVD F3,F0 the unmodified y is the return value
RET ,

So, the compiler operates on the value in memory without updating the value
in register F3.

Turning off registerization results in this code instead:

MOVD F0,y-8(SP) update y in memory
MOVW k-44(SP),R1 load k
CMP $-1021,R1, compare k to -1021
BLT ,10(PC) k < -1021, skip past RET below
MOVW k-44(SP),R1
SLL $20,R1 k << 20
MOVW y-4(SP),R2 load the high word of y in memory
ADD R1,R2 add k << 20 to it
MOVW R2,y-4(SP) store the result back to memory
MOVD y-8(SP),F0 load the modified y to be returned
NOP ,R0
RET ,

Is the problem with the compiler or with the C code? Although I get the
feeling that the compiler should figure out that the value of y is being
modified, I don't know if it's being done in a valid way, or a way that
compilers are expected to handle.

It could be an endianness-related thing: vc appears to do the right thing.
Having said that, so does 8c, as far as I can tell.

Any thoughts?

David