Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
Ok, thanks for the clarification jakub. Umesg On Mon, May 7, 2018, 2:08 PM Jakub Jelinek wrote: > On Mon, May 07, 2018 at 01:58:48PM +0530, Umesh Kalappa wrote: > > CCed Jakub, > > > > Agree that float division don't touch memory ,but fdiv result (stack > > > register ) is stored back to a memory i.e fResult . > > That doesn't really matter. It is stored to a stack spill slot, something > that doesn't have address taken and other code (e.g. in other threads) > can't > in a valid program access it. That is not considered memory for the > inline-asm, only objects that must live in memory count. > > Jakub >
Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
On Mon, May 07, 2018 at 01:58:48PM +0530, Umesh Kalappa wrote: > CCed Jakub, > > Agree that float division don't touch memory ,but fdiv result (stack > > register ) is stored back to a memory i.e fResult . That doesn't really matter. It is stored to a stack spill slot, something that doesn't have address taken and other code (e.g. in other threads) can't in a valid program access it. That is not considered memory for the inline-asm, only objects that must live in memory count. Jakub
Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
CCed Jakub, > Hi Alex, > Agree that float division don't touch memory ,but fdiv result (stack > register ) is stored back to a memory i.e fResult . > > So compiler barrier in the inline asm i.e ::memory should prevent the > shrinkage of instructions like "fstps fResult(%rip)"behind the > fence ? > > BTW ,if we make fDivident and fResult = 0.0f gloabls,the code > emitted looks ok i.e > #gcc -S test.c -O3 -mmmx -mno-sse > >flds.LC0(%rip) > fstsfDivident(%rip) > fdivs .LC1(%rip) > fstps fResult(%rip) > #APP > # 10 "test.c" 1 > mfence > # 0 "" 2 > #NO_APP > fldsfResult(%rip) > movl$.LC2, %edi > xorl%eax, %eax > fstpl (%rsp) > callprintf > > So i strongly believe that ,its compiler issue and please feel free > correct me in any case. > > Thank you and waiting for your reply. > > ~Umesh > > > > > On Fri, Apr 13, 2018 at 5:58 PM, Alexander Monakovwrote: >> On Fri, 13 Apr 2018, Vivek Kinhekar wrote: >>> The mfence instruction with memory clobber asm instruction should create a >>> barrier between division and printf instructions. >> >> No, floating-point division does not touch memory, so the asm does not (and >> need not) restrict its motion. >> >> Alexander
Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
Hi Alex , Agree that float division don't touch memory ,but fdiv result (stack register ) is stored back to a memory i.e fResult . So compiler barrier in the inline asm i.e ::memory should prevent the shrinkage of instructions like "fstps fResult(%rip)"behind the fence ? BTW ,if we make fDivident and fResult = 0.0f gloabls,the code emitted looks ok i.e #gcc -S test.c -O3 -mmmx -mno-sse flds.LC0(%rip) fstsfDivident(%rip) fdivs .LC1(%rip) fstps fResult(%rip) #APP # 10 "test.c" 1 mfence # 0 "" 2 #NO_APP fldsfResult(%rip) movl$.LC2, %edi xorl%eax, %eax fstpl (%rsp) callprintf So i strongly believe that ,its compiler issue and please feel free correct me in any case. Thank you and waiting for your reply. ~Umesh On Fri, Apr 13, 2018 at 5:58 PM, Alexander Monakovwrote: > On Fri, 13 Apr 2018, Vivek Kinhekar wrote: >> The mfence instruction with memory clobber asm instruction should create a >> barrier between division and printf instructions. > > No, floating-point division does not touch memory, so the asm does not (and > need not) restrict its motion. > > Alexander
Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
On Fri, Apr 13, 2018 at 01:34:21PM +, Vivek Kinhekar wrote: > Hello Alexander, > > In the given testcase, the generated fdivrs instruction performs the > division of a symbol ref (memory value) by FPU Stack Register and stores > the value in FPU Stack Register. The stack registers are not memory. > Please find the following RTL Dump of the fdivrs instruction generated. > It clearly access the memory for read access! That is a constant read, that doesn't count either. It is in memory only because the instruction doesn't support constant immediates, the memory is read-only. Jakub
RE: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
Oh! Thanks for the quick response, Jakub. Regards, Vivek Kinhekar -Original Message- From: Jakub Jelinek <ja...@redhat.com> Sent: Friday, April 13, 2018 7:08 PM To: Vivek Kinhekar <vivek.kinhe...@blackfigtech.com> Cc: Alexander Monakov <amona...@ispras.ru>; gcc@gcc.gnu.org Subject: Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction On Fri, Apr 13, 2018 at 01:34:21PM +, Vivek Kinhekar wrote: > Hello Alexander, > > In the given testcase, the generated fdivrs instruction performs the > division of a symbol ref (memory value) by FPU Stack Register and > stores the value in FPU Stack Register. The stack registers are not memory. > Please find the following RTL Dump of the fdivrs instruction generated. > It clearly access the memory for read access! That is a constant read, that doesn't count either. It is in memory only because the instruction doesn't support constant immediates, the memory is read-only. Jakub
RE: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
Hello Alexander, In the given testcase, the generated fdivrs instruction performs the division of a symbol ref (memory value) by FPU Stack Register and stores the value in FPU Stack Register. Please find the following RTL Dump of the fdivrs instruction generated. It clearly access the memory for read access! === #(insn:TI 13 20 16 2 (set (reg:XF 8 st) #(div:XF (float_extend:XF (mem/u/c:SF (symbol_ref/u:SI ("*.LC0") [flags 0x2]) [4 S4 A32])) #(reg:XF 8 st))) {*fop_xf_4_i387} # (nil)) fdivrs .LC0# 13*fop_xf_4_i387/1[length = 6] === Are we missing anything subtle here? Regards, Vivek Kinhekar -Original Message- From: Alexander Monakov <amona...@ispras.ru> Sent: Friday, April 13, 2018 5:58 PM To: Vivek Kinhekar <vivek.kinhe...@blackfigtech.com> Cc: gcc@gcc.gnu.org Subject: Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction On Fri, 13 Apr 2018, Vivek Kinhekar wrote: > The mfence instruction with memory clobber asm instruction should > create a barrier between division and printf instructions. No, floating-point division does not touch memory, so the asm does not (and need not) restrict its motion. Alexander
RE: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
Thanks for the quick response, Alexander! Regards, Vivek Kinhekar +91-7709046470 -Original Message- From: Alexander Monakov <amona...@ispras.ru> Sent: Friday, April 13, 2018 5:58 PM To: Vivek Kinhekar <vivek.kinhe...@blackfigtech.com> Cc: gcc@gcc.gnu.org Subject: Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction On Fri, 13 Apr 2018, Vivek Kinhekar wrote: > The mfence instruction with memory clobber asm instruction should > create a barrier between division and printf instructions. No, floating-point division does not touch memory, so the asm does not (and need not) restrict its motion. Alexander
Re: GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
On Fri, 13 Apr 2018, Vivek Kinhekar wrote: > The mfence instruction with memory clobber asm instruction should create a > barrier between division and printf instructions. No, floating-point division does not touch memory, so the asm does not (and need not) restrict its motion. Alexander
GCC Compiler Optimization ignores or mistreats MFENCE memory barrier related instruction
Hi, We are trying to create a memory barrier with following testcase. = #include void Test() { float fDivident = 0.1f; float fResult = 0.0f; fResult = ( fDivident / fResult ); __asm volatile ("mfence" ::: "memory"); printf("\nResult: %f\n", fResult); } == 'mfence' performs a serializing operation on all load-from-memory and store-to-memory instructions that were issued prior the MFENCE instruction. This serializing operation guarantees that every load and store instruction that precedes the MFENCE instruction in program order becomes globally visible before any load or store instruction that follows the MFENCE instruction. The mfence instruction with memory clobber asm instruction should create a barrier between division and printf instructions. When the testcase is compiled with optimization options O1 and above it can be observed that the mfence instruction is reordered and precedes division instruction. We expected that the two sets of assembly instructions, one pertaining to division operation and another pertaining to the printf operation, would not get mixed up on reordering by the GCC compiler optimizer because of the presence of the __asm volatile ("mfence" ::: "memory"); line between them. But, the generated assembly, which is inlined below for reference, isn't quite right as per our expectation. pushl %ebp# 23*pushsi2[length = 1] movl%esp, %ebp # 24*movsi_internal/1 [length = 2] subl$24, %esp # 25pro_epilogue_adjust_stack_si_add/1 [length = 3] mfence fldz# 20*movxf_internal/3 [length = 2] fdivrs .LC0# 13*fop_xf_4_i387/1[length = 6] You may note that the mfence instruction is generated before the fdivrs instruction. Can you please let us know if the usage of the "asm (mfence)" instruction as given in the above testcase is the right way of creating the expected memory barrier between the two sets of instructions pertaining to the division and printf operations, respectively or not? If yes, then we think, it's a bug in Compiler. Could you please confirm? If no, then what is the correct usage of "asm (mfence)" so as to get/ achieve the memory barrier functionality as expected in the above testcase? Thanks, Vivek Kinhekar