hfinkel added a comment.
In https://reviews.llvm.org/D39053#906513, @spetrovic wrote:
> Well, basically I'm just expanding the existing algorithm, why should we
> split fields just in case when current field is integer,
> I'm not resolving specific problem with unaligned loads/stores on MIPS.
>
> In this example:
>
> typedef struct {
>
> unsigned int f1 : 28;
> unsigned int f2 : 4;
> unsigned int f3 : 12;
>
> } S5;
>
> S5 *cmd;
>
> void foo() {
>
> cmd->f3 = 5;
>
> }
>
> With this patch there is improvement in code size not just on MIPS
> architecture, on X86 and ARM is also improved code size. If structure S5 is
> treated as i48 type there are extra instructions for reading it not just on
> MIPS. We can take results for MIPS just for example:
>
> Output without the patch:
>
> 0000000000000000 <foo>:
>
> 0: 3c010000 lui at,0x0
> 4: 0039082d daddu at,at,t9
> 8: 64210000 daddiu at,at,0
> c: dc210000 ld at,0(at)
> 10: dc210000 ld at,0(at)
> 14: 68220000 ldl v0,0(at)
> 18: 6c220007 ldr v0,7(at)
> 1c: 64030005 daddiu v1,zero,5
> 20: 7c62fd07 dins v0,v1,0x14,0xc
> 24: b0220000 sdl v0,0(at)
> 28: 03e00008 jr ra
> 2c: b4220007 sdr v0,7(at)
>
>
>
> Output with the patch:
>
> 0000000000000000 <foo>:
>
> 0: 3c010000 lui at,0x0
> 4: 0039082d daddu at,at,t9
> 8: 64210000 daddiu at,at,0
> c: dc210000 ld at,0(at)
> 10: dc210000 ld at,0(at)
> 14: 94220004 lhu v0,4(at)
> 18: 24030005 li v1,5
> 1c: 7c62f904 ins v0,v1,0x4,0x1c
> 20: 03e00008 jr ra
> 24: a4220004 sh v0,4(at)
>
>
> This is simple example, in more complicated examples there is more
> improvement.
I think this is part of the slippery slope we didn't want to go down. We
introduced this mode in the first place only to resolve a store-to-load
forwarding problem that is theoretically unsolvable by any local lowering
decisions. This, on the other hand, looks like a local code-generation problem
that we can/should fix in the backend. If I'm wrong, we should consider this as
well.
https://reviews.llvm.org/D39053
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