https://bugs.llvm.org/show_bug.cgi?id=40969
Bug ID: 40969
Summary: more compact sequence of x86 movs generated when using
-g
Product: new-bugs
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: greg.bedw...@sony.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
Blocks: 37728
In response to this llvm-dev post:
http://lists.llvm.org/pipermail/llvm-dev/2019-March/130779.html
Here is an example using the check-cfc tool (
https://github.com/llvm/llvm-project/tree/master/clang/utils/check_cfc ) to
spot a codegen difference depending on whether -g is specified or not. The
reduced source example is a little bit nasty as there's some dodgy looking
pointer casting in there, but regardless the absence or presence of -g should
not make a difference to generated code.
Here's my check_cfc.cfg file contents:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[Checks]
dash_g_no_change = true
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
and here it is in action:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$ clang -v
Check CFC, checking: dash_g_no_change
clang version 9.0.0 (trunk 355410) (llvm/trunk 355412)
Target: x86_64-scei-ps4
Thread model: posix
InstalledDir: E:\work\upstream-llvm\build-vs2015-ps4-ninja\bin
$ cat 1.cpp
using alpha = float __attribute__((ext_vector_type(4)));
void bravo(alpha charlie) {
unsigned char *delta = (unsigned char *)&charlie;
delta[0] = 0;
volatile alpha echo = charlie;
}
$ clang -c 1.cpp -O2 -g -fno-strict-aliasing --target=x86_64-unknown-unknown
Check CFC, checking: dash_g_no_change
1.cpp Code difference detected with -g
--- c:\tmp\gnochange\example\tmp\tmp_v5akp.o
+++ c:\tmp\gnochange\example\tmp\tmpvhabwu.o
@@ -5,6 +5,6 @@
0x00000001: 48 89 E5 mov rbp,rsp
0x00000004: 0F 29 45 F0 movaps xmmword ptr [rbp-10h],xmm0
0x00000008: C6 45 F0 00 mov byte ptr [rbp-10h],0
-0x0000000C: 0F 28 45 F0 movaps xmm0,xmmword ptr [rbp-10h]
-0x00000010: 0F 29 45 E0 movaps xmmword ptr [rbp-20h],xmm0
-0x00000014: 5D pop rbp
+0x0000000C: 8A 45 F0 mov al,byte ptr [rbp-10h]
+0x0000000F: 88 45 E0 mov byte ptr [rbp-20h],al
+0x00000012: 48 8B 45 F1 mov rax,qword ptr [rbp-0Fh]
*** Diff truncated ***
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Referenced Bugs:
https://bugs.llvm.org/show_bug.cgi?id=37728
[Bug 37728] [meta] Make llvm passes debug info invariant
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