diff options
| -rw-r--r-- | llvm/lib/Target/X86/README.txt | 103 |
1 files changed, 22 insertions, 81 deletions
diff --git a/llvm/lib/Target/X86/README.txt b/llvm/lib/Target/X86/README.txt index d4545a6fcfd..efc0cd82f23 100644 --- a/llvm/lib/Target/X86/README.txt +++ b/llvm/lib/Target/X86/README.txt @@ -1103,57 +1103,6 @@ be folded into: shl [mem], 1 //===---------------------------------------------------------------------===// -This testcase misses a read/modify/write opportunity (from PR1425): - -void vertical_decompose97iH1(int *b0, int *b1, int *b2, int width){ - int i; - for(i=0; i<width; i++) - b1[i] += (1*(b0[i] + b2[i])+0)>>0; -} - -We compile it down to: - -LBB1_2: # bb - movl (%esi,%edi,4), %ebx - addl (%ecx,%edi,4), %ebx - addl (%edx,%edi,4), %ebx - movl %ebx, (%ecx,%edi,4) - incl %edi - cmpl %eax, %edi - jne LBB1_2 # bb - -the inner loop should add to the memory location (%ecx,%edi,4), saving -a mov. Something like: - - movl (%esi,%edi,4), %ebx - addl (%edx,%edi,4), %ebx - addl %ebx, (%ecx,%edi,4) - -Here is another interesting example: - -void vertical_compose97iH1(int *b0, int *b1, int *b2, int width){ - int i; - for(i=0; i<width; i++) - b1[i] -= (1*(b0[i] + b2[i])+0)>>0; -} - -We miss the r/m/w opportunity here by using 2 subs instead of an add+sub[mem]: - -LBB9_2: # bb - movl (%ecx,%edi,4), %ebx - subl (%esi,%edi,4), %ebx - subl (%edx,%edi,4), %ebx - movl %ebx, (%ecx,%edi,4) - incl %edi - cmpl %eax, %edi - jne LBB9_2 # bb - -Additionally, LSR should rewrite the exit condition of these loops to use -a stride-4 IV, would would allow all the scales in the loop to go away. -This would result in smaller code and more efficient microops. - -//===---------------------------------------------------------------------===// - In SSE mode, we turn abs and neg into a load from the constant pool plus a xor or and instruction, for example: @@ -1301,15 +1250,8 @@ FirstOnet: xorl %eax, %eax ret -There are a few possible improvements here: -1. We should be able to eliminate the dead load into %ecx -2. We could change the "movl 8(%esp), %eax" into - "movzwl 10(%esp), %eax"; this lets us change the cmpl - into a testl, which is shorter, and eliminate the shift. - -We could also in theory eliminate the branch by using a conditional -for the address of the load, but that seems unlikely to be worthwhile -in general. +We could change the "movl 8(%esp), %eax" into "movzwl 10(%esp), %eax"; this +lets us change the cmpl into a testl, which is shorter, and eliminate the shift. //===---------------------------------------------------------------------===// @@ -1331,22 +1273,23 @@ bb7: ; preds = %entry to: -_foo: +foo: # @foo +# BB#0: # %entry + movl 4(%esp), %ecx cmpb $0, 16(%esp) - movl 12(%esp), %ecx + je .LBB0_2 +# BB#1: # %bb movl 8(%esp), %eax - movl 4(%esp), %edx - je LBB1_2 # bb7 -LBB1_1: # bb - addl %edx, %eax + addl %ecx, %eax ret -LBB1_2: # bb7 - movl %edx, %eax - subl %ecx, %eax +.LBB0_2: # %bb7 + movl 12(%esp), %edx + movl %ecx, %eax + subl %edx, %eax ret -The coalescer could coalesce "edx" with "eax" to avoid the movl in LBB1_2 -if it commuted the addl in LBB1_1. +There's an obviously unnecessary movl in .LBB0_2, and we could eliminate a +couple more movls by putting 4(%esp) into %eax instead of %ecx. //===---------------------------------------------------------------------===// @@ -1396,8 +1339,7 @@ Also check why xmm7 is not used at all in the function. //===---------------------------------------------------------------------===// -Legalize loses track of the fact that bools are always zero extended when in -memory. This causes us to compile abort_gzip (from 164.gzip) from: +Take the following: target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128" target triple = "i386-apple-darwin8" @@ -1416,16 +1358,15 @@ bb4.i: ; preds = %entry } declare void @exit(i32) noreturn nounwind -into: - -_abort_gzip: +This compiles into: +_abort_gzip: ## @abort_gzip +## BB#0: ## %entry subl $12, %esp movb _in_exit.4870.b, %al - notb %al - testb $1, %al - jne LBB1_2 ## bb4.i -LBB1_1: ## bb.i - ... + cmpb $1, %al + jne LBB0_2 + +We somehow miss folding the movb into the cmpb. //===---------------------------------------------------------------------===// |
