Hi, I just spotted this bit in Android's Makefiles (inherited that way from AOSP):
TARGET_arm_CFLAGS := [...] -fstrict-aliasing [...] [...] TARGET_thumb_CFLAGS := [...] -fno-strict-aliasing [...]
This general assumption that arm code can handle strict aliasing, but thumb code can't, seems odd.
My guess is that they started out using -fno-strict-aliasing everywhere, then fixed aliasing violations and "fixed" the compiler flags for arm, and didn't notice they're still turning it off for thumb -- either that, or they've run into toolchain bugs (that might be fixed in the Linaro toolchain), or it's a matter of "something we're building in thumb mode has aliasing violations and we're too lazy to figure out which part it is".
Unless someone knows of a valid reason for doing things the way they did, simply taking it out (preferrably replacing it with -Werror=strict-aliasing, so we're likely to notice any aliasing violations if it comes down to "too lazy to figure out which part") should get us some free extra speed. (Not sure just how much we're actually building in thumb mode - may or may not be relevant.)
ttyl bero
Bero,
I think this might be a good patch to get up to AOSP after we try it out. Assigning a bug for Chao to look at:
https://bugs.launchpad.net/linaro-android/+bug/822113
-Zach
On 2 August 2011 22:09, Bernhard Rosenkranzer bernhard.rosenkranzer@linaro.org wrote:
Hi, I just spotted this bit in Android's Makefiles (inherited that way from AOSP):
TARGET_arm_CFLAGS := [...] -fstrict-aliasing [...] [...] TARGET_thumb_CFLAGS := [...] -fno-strict-aliasing [...]
This general assumption that arm code can handle strict aliasing, but thumb code can't, seems odd.
My guess is that they started out using -fno-strict-aliasing everywhere, then fixed aliasing violations and "fixed" the compiler flags for arm, and didn't notice they're still turning it off for thumb -- either that, or they've run into toolchain bugs (that might be fixed in the Linaro toolchain), or it's a matter of "something we're building in thumb mode has aliasing violations and we're too lazy to figure out which part it is".
Unless someone knows of a valid reason for doing things the way they did, simply taking it out (preferrably replacing it with -Werror=strict-aliasing, so we're likely to notice any aliasing violations if it comes down to "too lazy to figure out which part") should get us some free extra speed. (Not sure just how much we're actually building in thumb mode - may or may not be relevant.)
ttyl bero
linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
-
Bernhard Rosenkranzer -
Zach Pfeffer