From 6ed53c27efc1b3eeb987b84b997633bc9456e5aa Mon Sep 17 00:00:00 2001 From: Ian Romanick Date: Mon, 10 Nov 2014 06:06:47 -0800 Subject: i965: Micro-optimize brw_get_index_type With the switch-statement, GCC 4.8.3 produces a small pile of code with a branch. 00000000 : 000000: 8b 54 24 04 mov 0x4(%esp),%edx 000004: b8 01 00 00 00 mov $0x1,%eax 000009: 81 fa 03 14 00 00 cmp $0x1403,%edx 00000f: 74 0d je 00001e 000011: 31 c0 xor %eax,%eax 000013: 81 fa 05 14 00 00 cmp $0x1405,%edx 000019: 0f 94 c0 sete %al 00001c: 01 c0 add %eax,%eax 00001e: c3 ret However, this could be two instructions. 00000000 : 000000: 2d 01 14 00 00 sub $0x1401,%eax 000005: d1 e8 shr %eax 000007: 90 nop 000008: 90 nop 000009: 90 nop 00000a: 90 nop 00000b: c3 ret The function was also moved to the header so that it could be inlined at the two call sites. Without this, 32-bit also needs to pull the parameter from the stack. This means there is a push, a call, a move, and a ret added to a two instruction function. The above code shows the function with __attribute__((regparm=1)), but even this adds several extra instructions. There is also an extra instruction on 64-bit to move the parameter to %eax for the subtract. On Bay Trail-D using Fedora 20 compile flags (-m64 -O2 -mtune=generic for 64-bit and -m32 -march=i686 -mtune=atom for 32-bit), affects Gl32Batch7: 32-bit: Difference at 95.0% confidence 0.818589% +/- 0.234661% (n=40) 64-bit: Difference at 95.0% confidence 0.54554% +/- 0.354092% (n=40) Signed-off-by: Ian Romanick Reviewed-by: Kenneth Graunke Reviewed-by: Matt Turner --- src/mesa/drivers/dri/i965/gen8_draw_upload.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'src/mesa/drivers/dri/i965/gen8_draw_upload.c') diff --git a/src/mesa/drivers/dri/i965/gen8_draw_upload.c b/src/mesa/drivers/dri/i965/gen8_draw_upload.c index 0d6feec8ab0..1af90ecc6a4 100644 --- a/src/mesa/drivers/dri/i965/gen8_draw_upload.c +++ b/src/mesa/drivers/dri/i965/gen8_draw_upload.c @@ -251,7 +251,7 @@ gen8_emit_index_buffer(struct brw_context *brw) BEGIN_BATCH(5); OUT_BATCH(CMD_INDEX_BUFFER << 16 | (5 - 2)); - OUT_BATCH(brw_get_index_type(index_buffer->type) << 8 | mocs_wb); + OUT_BATCH(brw_get_index_type(index_buffer->type) | mocs_wb); OUT_RELOC64(brw->ib.bo, I915_GEM_DOMAIN_VERTEX, 0, 0); OUT_BATCH(brw->ib.bo->size); ADVANCE_BATCH(); -- cgit v1.2.3