Optimisation: WRAM access with GlobalSymbol + Offset #5
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yberthier
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Jan 29, 2025
Recent shouldAssumeDSOLocal changes (introduced by 961f31d) do not take in consideration the relocation model anymore. The ARM fast-isel pass uses the function return to set whether a global symbol is loaded indirectly or not, and without the expected information llvm now generates an extra load for following code: ``` $ cat test.ll @__asan_option_detect_stack_use_after_return = external global i32 define dso_local i32 @main(i32 %argc, i8** %argv) #0 { entry: %0 = load i32, i32* @__asan_option_detect_stack_use_after_return, align 4 %1 = icmp ne i32 %0, 0 br i1 %1, label %2, label %3 2: ret i32 0 3: ret i32 1 } attributes #0 = { noinline optnone } $ lcc test.ll -o - [...] main: .fnstart [...] movw r0, :lower16:__asan_option_detect_stack_use_after_return movt r0, :upper16:__asan_option_detect_stack_use_after_return ldr r0, [r0] ldr r0, [r0] cmp r0, #0 [...] ``` And without 'optnone' it produces: ``` [...] main: .fnstart [...] movw r0, :lower16:__asan_option_detect_stack_use_after_return movt r0, :upper16:__asan_option_detect_stack_use_after_return ldr r0, [r0] clz r0, r0 lsr r0, r0, #5 bx lr [...] ``` This triggered a lot of invalid memory access in sanitizers for arm-linux-gnueabihf. I checked this patch both a stage1 built with gcc and a stage2 bootstrap and it fixes all the Linux sanitizers issues. Reviewed By: MaskRay Differential Revision: https://reviews.llvm.org/D95379
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Memory access to GlobalAddress + Immediate Offset can be optimised (by using Immediate expressions to wrap the GlobalAddress + Immediate Offset, which will be handled by the linker).
See this example where we could remove the
moveinstruction and have this load instruction:lw r21, zero, my_table + 4The text was updated successfully, but these errors were encountered: