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Fork of llvm with experimental commits and workarounds for RPCS3
16d66c3d22
The motivation for this patch starts with PR20134: https://llvm.org/bugs/show_bug.cgi?id=20134 void foo(int *a, int i) { a[i] = a[i+1] + a[i+2]; } It seems better to produce this (14 bytes): movslq %esi, %rsi movl 0x4(%rdi,%rsi,4), %eax addl 0x8(%rdi,%rsi,4), %eax movl %eax, (%rdi,%rsi,4) Rather than this (22 bytes): leal 0x1(%rsi), %eax cltq leal 0x2(%rsi), %ecx movslq %ecx, %rcx movl (%rdi,%rcx,4), %ecx addl (%rdi,%rax,4), %ecx movslq %esi, %rax movl %ecx, (%rdi,%rax,4) The most basic problem (the first test case in the patch combines constants) should also be fixed in InstCombine, but it gets more complicated after that because we need to consider architecture and micro-architecture. For example, AArch64 may not see any benefit from the more general transform because the ISA solves the sexting in hardware. Some x86 chips may not want to replace 2 ADD insts with 1 LEA, and there's an attribute for that: FeatureSlowLEA. But I suspect that doesn't go far enough or maybe it's not getting used when it should; I'm also not sure if FeatureSlowLEA should also mean "slow complex addressing mode". I see no perf differences on test-suite with this change running on AMD Jaguar, but I see small code size improvements when building clang and the LLVM tools with the patched compiler. A more general solution to the sext(add nsw(x, C)) problem that works for multiple targets is available in CodeGenPrepare, but it may take quite a bit more work to get that to fire on all of the test cases that this patch takes care of. Differential Revision: http://reviews.llvm.org/D13757 llvm-svn: 250560 |
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