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evaluate the LHS or the RHS of an operation first. This causes good things
to happen. For example, instead of compiling a loop to this:
.LBBstrength_result7_1: # loopentry
movl 16(%esp), %edi
movl (%edi), %edi ;;; LOAD
movl (%ecx), %ebx
movl $2, (%eax,%ebx,4)
movl (%edx), %ebx
movl %esi, %ebp
addl $21, %ebp
addl $42, %esi
cmpl $0, %edi ;;; USE
cmovne %esi, %ebp
cmpl %ebp, %ebx
movl %ebp, %esi
jg .LBBstrength_result7_1
We now compile it to this:
.LBBstrength_result7_1: # loopentry
movl %edi, %ebx
addl $42, %ebx
addl $21, %edi
movl (%ecx), %ebp ;; LOAD
cmpl $0, %ebp ;; USE
cmovne %ebx, %edi
movl (%edx), %ebx
movl $2, (%eax,%ebx,4)
movl (%esi), %ebx
cmpl %edi, %ebx
jg .LBBstrength_result7_1
Which reduces register pressure enough (in this case) to avoid spilling in the
loop.
As another example, consider the CodeGen/X86/regpressure.ll testcase. We
used to generate this code for both cases:
regpressure1:
subl $32, %esp
movl %esi, 12(%esp)
movl %edi, 8(%esp)
movl %ebx, 4(%esp)
movl %ebp, (%esp)
movl 36(%esp), %ecx
movl (%ecx), %eax
movl 4(%ecx), %edx
movl %edx, 24(%esp)
movl 8(%ecx), %edx
movl %edx, 16(%esp)
movl 12(%ecx), %edx
movl 16(%ecx), %esi
movl 20(%ecx), %edi
movl 24(%ecx), %ebx
movl %ebx, 28(%esp)
movl 28(%ecx), %ebx
movl 32(%ecx), %ebp
movl %ebp, 20(%esp)
movl 36(%ecx), %ecx
imull 24(%esp), %eax
imull 16(%esp), %eax
imull %edx, %eax
imull %esi, %eax
imull %edi, %eax
imull 28(%esp), %eax
imull %ebx, %eax
imull 20(%esp), %eax
imull %ecx, %eax
movl (%esp), %ebp
movl 4(%esp), %ebx
movl 8(%esp), %edi
movl 12(%esp), %esi
addl $32, %esp
ret
This code is basically trying to do all of the loads first, then execute all
of the multiplies. Because we run out of registers, lots of spill code happens.
We now generate this code for both cases:
regpressure1:
movl 4(%esp), %ecx
movl (%ecx), %eax
movl 4(%ecx), %edx
imull %edx, %eax
movl 8(%ecx), %edx
imull %edx, %eax
movl 12(%ecx), %edx
imull %edx, %eax
movl 16(%ecx), %edx
imull %edx, %eax
movl 20(%ecx), %edx
imull %edx, %eax
movl 24(%ecx), %edx
imull %edx, %eax
movl 28(%ecx), %edx
imull %edx, %eax
movl 32(%ecx), %edx
imull %edx, %eax
movl 36(%ecx), %ecx
imull %ecx, %eax
ret
which is much nicer (when we fold loads into the muls it will be even better).
The old instruction selector used to produce the good code for regpressure1
but not for regpressure2, as it depended on the order of operations in the
LLVM code.
llvm-svn: 19449
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