Chris Lattner 6bd6da4097 Be far more careful when splitting a loop header, either to form a preheader ... or when splitting loops with a common header into multiple loops. In particular the old code would always insert the preheader before the old loop header. This is disasterous in cases where the loop hasn't been rotated. For example, it can produce code like: .. outside the loop... jmp LBB1_2 #bb13.outer LBB1_1: #bb1 movsd 8(%esp,%esi,8), %xmm1 mulsd (%edi), %xmm1 addsd %xmm0, %xmm1 addl $24, %edi incl %esi jmp LBB1_3 #bb13 LBB1_2: #bb13.outer leal (%edx,%eax,8), %edi pxor %xmm1, %xmm1 xorl %esi, %esi LBB1_3: #bb13 movapd %xmm1, %xmm0 cmpl $4, %esi jl LBB1_1 #bb1 Note that the loop body is actually LBB1_1 + LBB1_3, which means that the loop now contains an uncond branch WITHIN it to jump around the inserted loop header (LBB1_2). Doh. This patch changes the preheader insertion code to insert it in the right spot, producing this code: ... outside the loop, fall into the header ... LBB1_1: #bb13.outer leal (%edx,%eax,8), %esi pxor %xmm0, %xmm0 xorl %edi, %edi jmp LBB1_3 #bb13 LBB1_2: #bb1 movsd 8(%esp,%edi,8), %xmm0 mulsd (%esi), %xmm0 addsd %xmm1, %xmm0 addl $24, %esi incl %edi LBB1_3: #bb13 movapd %xmm0, %xmm1 cmpl $4, %edi jl LBB1_2 #bb1 Totally crazy, no branch in the loop! :) llvm-svn: 30587		2006-09-23 08:19:21 +00:00
llvm	Be far more careful when splitting a loop header, either to form a preheader	2006-09-23 08:19:21 +00:00