937944 - Make ARM's scratch float register different from its argument registers

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, defect)

Description

[Dan Gohman [:sunfish]]

Attachment: arm-scratch-reg.patch

The ARM backend currently uses d1 as both a scratch float register and an argument register. This makes it difficult for register allocators to keep an argument in a register.

Also, in bug 880512, the backtracking allocator is currently assuming that argument registers are allocatable, and the scratch register is not allocatable.

Attached is a patch which changes the scratch register from d1 to d14, which is not currently being used for any other special purpose.

Comment 1

[Marty Rosenberg [:mjrosenb]]

Comment on attachment 831196 [details] [diff] [review]
arm-scratch-reg.patch

Review of attachment 831196 [details] [diff] [review]:
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I look forward to seeing if this gives us a performance boost!

Comment 2

[Dan Gohman [:sunfish]]

Attachment: arm-scratch-reg.patch

This patch fixes a bug in the previous patch and makes a minor adjustment.

The bug was that I had removed d14 from the NonVolatileMask. I believe this is incorrect, because that register still needs to be saved and restored with the other non-volatile registers. This patch fixes that.

The minor adjustment was that I picked d15 instead of d14 to be the scratch register, and I moved NANRreg to d14. Since NANReg is only used in asm.js mode, this change makes the allocatable register set contiguous in all cases, which is nice.

Comment 3

[Dan Gohman [:sunfish]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/1fb0f60a3d6b

Comment 4

[:Ms2ger (he/him; ⌚ UTC+1/+2)]

https://hg.mozilla.org/mozilla-central/rev/1fb0f60a3d6b