I'm producing MIPS code with gcc 4.3.3, specifically on the PSP. When I started looking over the output, I started noticing a strange pattern: the code is often using the s registers which need to be saved rather than the t registers which don't. This produces code that constantly needs to save and load the s registers for no discernible reason -- a huge waste of cycles!
This happens regardless of the optimization level (though I haven't tried O0) Here's an example. Notice that only t0 is used, and the compiler chooses to use all the s registers instead. Notice how inefficient the code becomes: 089033e4 <_ZN9PspMemory8testCopyEPKhS1_j>: 89033e4: 27bdffd8 addiu sp,sp,-40 89033e8: afb60018 sw s6,24(sp) 89033ec: 00a0b021 move s6,a1 89033f0: 3c0508ac lui a1,0x8ac 89033f4: afb7001c sw s7,28(sp) 89033f8: afb40010 sw s4,16(sp) 89033fc: 0080b821 move s7,a0 8903400: 00c0a021 move s4,a2 8903404: 24a595c8 addiu a1,a1,-27192 8903408: 24040001 li a0,1 890340c: afbf0024 sw ra,36(sp) 8903410: afbe0020 sw s8,32(sp) 8903414: afb50014 sw s5,20(sp) 8903418: afb3000c sw s3,12(sp) 890341c: afb20008 sw s2,8(sp) 8903420: afb10004 sw s1,4(sp) 8903424: 0e242f24 jal 890bc90 <_Z13PspDebugTracebPKcz> 8903428: afb00000 sw s0,0(sp) 890342c: 12800036 beqz s4,8903508 <_ZN9PspMemory8testCopyEPKhS1_j+0x124> 8903430: 3c0208ac lui v0,0x8ac 8903434: 2455961c addiu s5,v0,-27108 8903438: 3c0208ac lui v0,0x8ac 890343c: 245e95fc addiu s8,v0,-27140 8903440: 02e09021 move s2,s7 8903444: 02c08821 move s1,s6 8903448: 00009821 move s3,zero 890344c: 0a240d1e j 8903478 <_ZN9PspMemory8testCopyEPKhS1_j+0x94> 8903450: 00008021 move s0,zero 8903454: 02003021 move a2,s0 8903458: 24040001 li a0,1 890345c: 0e242f24 jal 890bc90 <_Z13PspDebugTracebPKcz> 8903460: 02a02821 move a1,s5 8903464: 26100001 addiu s0,s0,1 8903468: 0214102b sltu v0,s0,s4 890346c: 26520001 addiu s2,s2,1 8903470: 10400015 beqz v0,89034c8 <_ZN9PspMemory8testCopyEPKhS1_j+0xe4> 8903474: 26310001 addiu s1,s1,1 8903478: 92480000 lbu t0,0(s2) 890347c: 92270000 lbu a3,0(s1) 8903480: 5107fff9 beql t0,a3,8903468 <_ZN9PspMemory8testCopyEPKhS1_j+0x84> 8903484: 26100001 addiu s0,s0,1 8903488: 1660fff3 bnez s3,8903458 <_ZN9PspMemory8testCopyEPKhS1_j+0x74> 890348c: 02003021 move a2,s0 8903490: 3c0508ac lui a1,0x8ac 8903494: 24040001 li a0,1 8903498: 0e242f24 jal 890bc90 <_Z13PspDebugTracebPKcz> 890349c: 24a595dc addiu a1,a1,-27172 89034a0: 02c03821 move a3,s6 89034a4: 02804021 move t0,s4 89034a8: 24040001 li a0,1 89034ac: 03c02821 move a1,s8 89034b0: 0e242f24 jal 890bc90 <_Z13PspDebugTracebPKcz> 89034b4: 02e03021 move a2,s7 89034b8: 24130001 li s3,1 89034bc: 92270000 lbu a3,0(s1) 89034c0: 0a240d15 j 8903454 <_ZN9PspMemory8testCopyEPKhS1_j+0x70> 89034c4: 92480000 lbu t0,0(s2) 89034c8: 1260000f beqz s3,8903508 <_ZN9PspMemory8testCopyEPKhS1_j+0x124> 89034cc: 3c0508ac lui a1,0x8ac 89034d0: 8fbf0024 lw ra,36(sp) 89034d4: 8fbe0020 lw s8,32(sp) 89034d8: 8fb7001c lw s7,28(sp) 89034dc: 8fb60018 lw s6,24(sp) 89034e0: 8fb50014 lw s5,20(sp) 89034e4: 8fb40010 lw s4,16(sp) 89034e8: 8fb3000c lw s3,12(sp) 89034ec: 8fb20008 lw s2,8(sp) 89034f0: 8fb10004 lw s1,4(sp) 89034f4: 8fb00000 lw s0,0(sp) 89034f8: 24a59c8c addiu a1,a1,-25460 89034fc: 24040001 li a0,1 8903500: 0a242f24 j 890bc90 <_Z13PspDebugTracebPKcz> 8903504: 27bd0028 addiu sp,sp,40 8903508: 3c0508ac lui a1,0x8ac 890350c: 8fbf0024 lw ra,36(sp) 8903510: 8fbe0020 lw s8,32(sp) 8903514: 8fb7001c lw s7,28(sp) 8903518: 8fb60018 lw s6,24(sp) 890351c: 8fb50014 lw s5,20(sp) 8903520: 8fb40010 lw s4,16(sp) 8903524: 8fb3000c lw s3,12(sp) 8903528: 8fb20008 lw s2,8(sp) 890352c: 8fb10004 lw s1,4(sp) 8903530: 8fb00000 lw s0,0(sp) 8903534: 24a59628 addiu a1,a1,-27096 8903538: 24040001 li a0,1 890353c: 0a242f24 j 890bc90 <_Z13PspDebugTracebPKcz> 8903540: 27bd0028 addiu sp,sp,40 -- View this message in context: http://old.nabble.com/GCC-for-MIPS-not-using-the-right-registers--tp29293261p29293261.html Sent from the gcc - bugs mailing list archive at Nabble.com.
