[PATCH] D32352: Go to eleven

[Lama via Phabricator via llvm-commits]

lsaba added inline comments.


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Comment at: test/CodeGen/X86/mul-constant-i16.ll:263
+; X64-NEXT:    addl %edi, %eax
 ; X64-NEXT:    # kill: %AX<def> %AX<kill> %EAX<kill>
 ; X64-NEXT:    retq
----------------
need to be careful when replacing one mul instruction with 2 lea instructions + other instructions, in the general case this should be faster, but if the RA chooses RBP/R13/EBP as the base register of the lea instruction  (instead of rdi in this case), the lea will be a slow lea so this sequence: 
leal (%r13,%r13,2), %eax
leal (%r13,%rax,4),%eax 
addl %r13d, %eax

will be slower than the mul instruction. 

patch https://reviews.llvm.org/D32277 will attempt to replace this sequence with 
leal (,%r13,2), %eax
add %r13,%eax
leal (,%rax,4),%eax 
add %r13,%eax
addl %r13d, %eax

which will cost ~the same as the mul instruction but with increase in code size.


https://reviews.llvm.org/D32352