[LLVMbugs] [Bug 12094] New: memcpy from float* should generate aligned load

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12094

             Bug #: 12094
           Summary: memcpy from float* should generate aligned load
           Product: clang
           Version: 3.0
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: jay.foad at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


In the example below, a memcpy from "float *p" is turned into an unaligned
load. I think it should be an *aligned* load, because I think clang can assume
that p is suitably aligned.

C99 (Committee Draft -- April 12, 2011) 6.3.2.3-7 says "A pointer to an object
type may be converted to a pointer to a different object type. If the resulting
pointer is not correctly aligned for the referenced type, the behavior is
undefined."


$ cat m.c
#include <string.h>
int f(float *p) {
    int i;
    memcpy(&i, p, sizeof i);
    return i;
}
$ clang -O2 -S -emit-llvm m.c -o -
; ModuleID = 'm.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define i32 @f(float* nocapture %p) nounwind uwtable readonly {
entry:
  %0 = bitcast float* %p to i32*
  %srcval = load i32* %0, align 1
  ret i32 %srcval
}

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