On 5/3/21 6:31 PM, Peng Yu via Gcc-help wrote:
In the following example, printf() is converted to puts(). What are
the options to turn off any possible optimization during compilation
and just let gcc literally translate the C code to the assembly code?
Note that clang does not perform such an optimization. Should gcc do
the same to turn off any optimization by default? Thanks.
Someone already answered the first question so just to address
the second: IMO, since these are optimizing transformations, without
optimization GCC should avoid introducing calls to library functions
that aren't in the source code. The possible exceptions are calling
memcpy/memmove/memset to copy or initialize large aggregates, and
runtime library calls.
Martin
$ gcc -S -x c -o - - <<< '#include <stdio.h>'$'\n''int main() {
puts("Hello World1!"); printf("Hello World2!\n"); }'
.file ""
.text
.section .rodata
.LC0:
.string "Hello World1!"
.LC1:
.string "Hello World2!"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
leaq .LC0(%rip), %rdi
call puts@PLT
leaq .LC1(%rip), %rdi
call puts@PLT
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Debian 10.2.1-6) 10.2.1 20210110"
.section .note.GNU-stack,"",@progbits
$ clang -S -x c -o - - <<< '#include <stdio.h>'$'\n''int main() {
puts("Hello World1!"); printf("Hello World2!\n"); }'
.text
.file "-"
.globl main # -- Begin function main
.p2align 4, 0x90
.type main,@function
main: # @main
.cfi_startproc
# %bb.0:
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
subq $16, %rsp
movabsq $.L.str, %rdi
callq puts
movabsq $.L.str.1, %rdi
movl %eax, -4(%rbp) # 4-byte Spill
movb $0, %al
callq printf
xorl %ecx, %ecx
movl %eax, -8(%rbp) # 4-byte Spill
movl %ecx, %eax
addq $16, %rsp
popq %rbp
.cfi_def_cfa %rsp, 8
retq
.Lfunc_end0:
.size main, .Lfunc_end0-main
.cfi_endproc
# -- End function
.type .L.str,@object # @.str
.section .rodata.str1.1,"aMS",@progbits,1
.L.str:
.asciz "Hello World1!"
.size .L.str, 14
.type .L.str.1,@object # @.str.1
.L.str.1:
.asciz "Hello World2!\n"
.size .L.str.1, 15
.ident "Debian clang version 11.0.1-2"
.section ".note.GNU-stack","",@progbits
.addrsig
.addrsig_sym puts
.addrsig_sym printf
